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COVER PAGE - shares	6 Months Ended
COVER PAGE - shares	Jun. 30, 2020	Aug. 04, 2020
Cover [Abstract]
Document Type	10-Q
Document Quarterly Report	true
Document Period End Date	Jun. 30, 2020
Document Transition Report	false
Entity File Number	001-35358
Entity Registrant Name	TC PipeLines, LP
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	52-2135448
Entity Address, Address Line One	700 Louisiana Street,
Entity Address, Address Line Two	Suite 700
Entity Address, City or Town	Houston,
Entity Address, State or Province	TX
Entity Address, Postal Zip Code	77002-2761
City Area Code	877
Local Phone Number	290-2772
Entity Current Reporting Status	Yes
Title of 12(b) Security	Common units representing limited partner interests
Trading Symbol	TCP
Security Exchange Name	NYSE
Entity Interactive Data Current	Yes
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		71,306,396
Entity Central Index Key	0001075607
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	Q2
Amendment Flag	false

CONSOLIDATED STATEMENTS OF INCOME - USD ($) shares in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Income Statement [Abstract]
Transmission revenues	$ 95	$ 93	$ 196	$ 206
Equity earnings (Note 5)	29	30	84	84
Operation and maintenance expenses	(16)	(17)	(32)	(33)
Property taxes	(7)	(6)	(13)	(13)
General and administrative	(2)	(2)	(3)	(4)
Depreciation and amortization	(19)	(19)	(39)	(39)
Financial charges and other (Note 15)	(18)	(21)	(37)	(43)
Net income before taxes	62	58	156	158
Income taxes	(1)	(1)	(1)	(1)
Net income	61	57	155	157
Net income attributable to non-controlling interest	4	2	10	9
Net income attributable to controlling interests	57	55	145	148
Net income attributable to controlling interest allocation (Note 9)
Common units	56	54	142	145
General Partner	1	1	3	3
Net income attributable to controlling interests	$ 57	$ 55	$ 145	$ 148
Net income per common unit (Note 9) - basic and diluted (in dollars per unit)	$ 0.78	$ 0.75	$ 1.99	$ 2.03
Weighted average common units outstanding — basic and diluted (millions)	71.3	71.3	71.3	71.3
Common units outstanding, end of period (millions)	71.3	71.3	71.3	71.3

CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2018	Jun. 30, 2020	Jun. 30, 2019
Statement of Comprehensive Income [Abstract]
Net income	$ 61	$ 57	$ 155	$ 157
Other comprehensive income
Change in fair value of cash flow hedges (Note 13)	(2)	(9)	(15)	(14)
Reclassification to net income of (gains) and losses on cash flow hedges (Note 13)	2	1	2	1
Comprehensive income	61	49	142	144
Comprehensive income attributable to non-controlling interests	4	2	10	9
Comprehensive income attributable to controlling interests	$ 57	$ 47	$ 132	$ 135

- Definition

The consolidated profit or loss for the period, net of income taxes, including the portion attributable to the noncontrolling interest.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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+ Details

CONSOLIDATED BALANCE SHEETS - USD ($) $ in Millions	Jun. 30, 2020	Dec. 31, 2019
Current Assets
Cash and cash equivalents	$ 215	$ 83
Accounts receivable and other (Note 14)	34	43
Distribution receivable from Iroquois	0	14
Inventories	10	10
Other	3	6
Total current assets	262	156
Equity investments (Note 5)	1,082	1,098
Property, plant and equipment (Net of $1,218 accumulated depreciation; 2019 - $1,187)	1,598	1,528
Goodwill	71	71
TOTAL ASSETS	3,013	2,853
Current Liabilities
Accounts payable and accrued liabilities	49	28
Accounts payable to affiliates (Note 12)	6	8
Accrued interest	10	11
Current portion of long-term debt (Note 7)	350	123
Total current liabilities	415	170
Long-term debt, net (Note 7)	1,762	1,880
Deferred state income taxes	6	7
Other liabilities	43	36
Total liabilities	2,226	2,093
Partners’ Equity
General partner	15	14
Accumulated other comprehensive income (loss) (AOCI)	(18)	(5)
Controlling interests	685	656
Non-controlling interests	102	104
Total partners' equity	787	760
TOTAL LIABILITIES AND PARTNERS’ EQUITY	3,013	2,853
Common Units
Partners’ Equity
Limited partner	593	544
Class B Units
Partners’ Equity
Limited partner	$ 95	$ 103

CONSOLIDATED BALANCE SHEETS (Parenthetical) - USD ($) $ in Millions	Jun. 30, 2020	Dec. 31, 2019
Statement of Financial Position [Abstract]
Accumulated depreciation	$ 1,218	$ 1,187

CONSOLIDATED STATEMENT OF CASH FLOWS - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Cash Generated from Operations
Net income	$ 61.0	$ 57.0	$ 155.0	$ 157.0
Depreciation and amortization	19.0	19.0	39.0	39.0
Amortization of debt issue costs reported as interest expense			1.0	1.0
Equity earnings from equity investments (Note 5)	(29.0)	(30.0)	(84.0)	(84.0)
Distributions received from operating activities of equity investments (Note 5)			115.0	112.0
Equity allowance for funds used during construction			(3.0)	(1.0)
Change in operating working capital (Note 11)			6.0	4.0
Other			(1.0)	0.0
Cash Generated from Operations			228.0	228.0
Investing Activities
Proceeds from return of investment			5.0	55.0
Capital expenditures			(87.0)	(29.0)
Customer advances for construction			(1.0)
Customer advances for construction				1.0
Investing Activities			(88.0)	22.0
Financing Activities
Distributions paid to common units, including the General Partner (Note 10)			(95.0)	(95.0)
Distributions paid to Class B units (Note 8)			(8.0)	(13.0)
Distributions paid to non-controlling interests			(12.0)	(15.0)
Long-term debt issued, net of discount (Note 7)			207.0	20.0
Long-term debt repaid (Note 7)			(100.0)	(135.0)
Financing Activities			(8.0)	(238.0)
Increase in cash and cash equivalents			132.0	12.0
Cash and cash equivalents, beginning of period			83.0	33.0
Cash and cash equivalents, end of period	215.0	45.0	215.0	45.0
Great Lakes
Cash Generated from Operations
Equity earnings from equity investments (Note 5)	(9.0)	(9.0)	(29.0)	(29.0)
Investing Activities
Investments in equity investments			(5.0)	(5.0)
Iroquois
Cash Generated from Operations
Equity earnings from equity investments (Note 5)	(7.0)	(7.0)	(20.0)	(20.0)
Distributions received from operating activities of equity investments (Note 5)	10.0	14.0	38.0	28.0
Investing Activities
Proceeds from return of investment			5.0	5.0
Northern Border
Cash Generated from Operations
Equity earnings from equity investments (Note 5)	$ (13.0)	$ (14.0)	(35.0)	(35.0)
Investing Activities
Proceeds from return of investment			$ 0.0	$ 50.0

CONSOLIDATED STATEMENT OF CHANGES IN PARTNERS' EQUITY - 6 months ended Jun. 30, 2020 - USD ($)
shares in Millions, $ in Millions

Total

Non- Controlling Interest

Accumulated Other Comprehensive Income (Loss)

^[1]

Limited Partners

Common Units

Limited Partners

Class B Units

General Partner

Partners' Equity at beginning of period (in units) at Dec. 31, 2019

71.3

1.9

Partners' Equity at beginning of period at Dec. 31, 2019

$ 760

$ 104

$ (5)

$ 544

$ 103

$ 14

Increase (Decrease) in Partners' Equity

Net income

155

142

Other comprehensive income (loss)

(13)

Distributions

(115)

(12)

$ (93)

$ (8)

(2)

Partners' Equity at end of period (in units) at Jun. 30, 2020

71.3

1.9

Partners' Equity at end of period at Jun. 30, 2020

$ 787

$ 102

$ (18)

$ 593

$ 95

$ 15

[1]

Gain (loss) related to cash flow hedges reported in AOCI and expected to be reclassified to Net income in the next 12 months is estimated to be $(8) million. These estimates assume constant interest rates over time; however, the amounts reclassified will vary based on actual value of interest rates at the date of settlement.

- Definition

The consolidated profit or loss for the period, net of income taxes, including the portion attributable to the noncontrolling interest.

+ References

+ Details

CONSOLIDATED STATEMENT OF CHANGES IN PARTNERS' EQUITY (Parenthetical) $ in Millions	6 Months Ended
	Jun. 30, 2020 USD ($)
Statement of Partners' Capital [Abstract]
Gain (loss) related to cash flow hedges in AOCI expected to be reclassified to Net income in the next 12 months	$ (8)

ORGANIZATION	6 Months Ended
ORGANIZATION	Jun. 30, 2020
Limited Liability Company or Limited Partnership, Business Organization and Operations [Abstract]
ORGANIZATION	ORGANIZATIONTC PipeLines, LP and its subsidiaries are collectively referred to herein as the Partnership. The Partnership was formed by TransCanada PipeLines Limited, a wholly owned subsidiary of TC Energy Corporation (TC Energy Corporation together with its subsidiaries collectively referred to herein as TC Energy), to acquire, own and participate in the management of energy infrastructure assets in North America.

SIGNIFICANT ACCOUNTING POLICIES	6 Months Ended
SIGNIFICANT ACCOUNTING POLICIES	Jun. 30, 2020
Accounting Policies [Abstract]
SIGNIFICANT ACCOUNTING POLICIES	SIGNIFICANT ACCOUNTING POLICIES The accompanying consolidated financial statements and related notes have been prepared in accordance with United States generally accepted accounting principles (GAAP) and amounts are stated in U.S. dollars. The results of operations for the three and six months ended June 30, 2020 and 2019 are not necessarily indicative of the results that may be expected for the full fiscal year. The accompanying consolidated financial statements should be read in conjunction with the audited consolidated financial statements and notes thereto for the year ended December 31, 2019 included in our 2019 Annual Report. That report contains a more comprehensive summary of the Partnership’s significant accounting policies. In the opinion of management, the accompanying consolidated financial statements contain all of the appropriate adjustments, which are normally recurring adjustments unless otherwise noted, and considered necessary to present fairly the financial position of the Partnership, the results of operations and cash flows for the respective periods. Our significant accounting policies are consistent with those disclosed in our 2019 Annual Report, except those that became effective in 2020 as described in full under Note 3, "Accounting Pronouncements." Basis of Presentation The Partnership consolidates its interests in entities over which it is able to exercise control. To the extent there are interests owned by other parties, these interests are included as non-controlling interests. The Partnership uses the equity method of accounting for its investments in entities over which it is able to exercise significant influence. U.S. federal and certain state income taxes are the responsibility of the limited partners and are not reflected in these consolidated financial statements. The tax effect of the Partnership's activities accrues to its limited partners. The Partnership’s taxable income or loss, which may vary substantially from the net income or loss reported in the consolidated statement of operations, is includable in the U.S. federal income tax returns of each partner. In instances where the Partnership’s consolidated entities are subject to state income taxes, the asset-liability method is used to account for taxes. This method requires recognition of deferred tax assets and liabilities for future tax consequences attributable to the differences between the financial statement carrying amount of existing assets and liabilities and their respective tax bases. Deferred tax assets and liabilities are classified as non-current on our consolidated balance sheets. Use of Estimates The preparation of financial statements requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities, if any, as of the date of the financial statements, and the reported amounts of revenues and expenses during the reporting period. Although management believes these estimates are reasonable, actual results could differ from these estimates.

ACCOUNTING PRONOUNCEMENTS	6 Months Ended
ACCOUNTING PRONOUNCEMENTS	Jun. 30, 2020
Accounting Changes and Error Corrections [Abstract]
ACCOUNTING PRONOUNCEMENTS	ACCOUNTING PRONOUNCEMENTS Changes in Accounting Policies effective January 1, 2020 Measurement of credit losses on financial instruments In June 2016, the Financial Accounting Standards Board (FASB) issued new guidance that changes how entities measure credit losses for most financial assets and certain other financial instruments that are not measured at fair value through net income (loss). The new guidance amends the impairment model of financial instruments basing it on expected losses rather than incurred losses. These expected credit losses will be recognized as an allowance rather than as a direct write down of the amortized cost basis. The new guidance became effective January 1, 2020 and was applied using a modified retrospective approach. The adoption of this new guidance did not have a material impact on the Partnership’s consolidated financial statements. Consolidation In October 2018, the FASB issued new guidance for determining whether fees paid to decision makers and service providers are variable interests for indirect interests held through related parties under common control. This new guidance became effective January 1, 2020, and was applied on a retrospective basis. The adoption of this new guidance did not have a material impact on the Partnership’s consolidated financial statements. Reference rate reform In March 2020, in response to the expected cessation of LIBOR, the FASB issued new optional guidance that eases the potential burden of accounting for reference rate reform. The new guidance provides optional expedients for contracts and hedging relationships that are affected by reference rate reform, if certain criteria are met. Each of the expedients can be applied as of January 1, 2020 through December 31, 2022. For eligible hedging relationships existing as of January 1, 2020 and prospectively, the Partnership has applied the optional expedient allowing an entity to assume that the hedged forecasted transaction in a cash flow hedge is probable of occurring. As reference rate reform is still an ongoing process, the Partnership will continue to evaluate the timing and potential impact of adoption of other optional expedients when deemed necessary.

GOODWILL	6 Months Ended
GOODWILL	Jun. 30, 2020
Goodwill and Intangible Assets Disclosure [Abstract]
GOODWILL	GOODWILL Under U.S. GAAP, we evaluate our goodwill related to Tuscarora and North Baja for impairment at least annually and if any indicators of impairment are evident. In 2019, based on our analysis of Tuscarora and North Baja’s current market conditions, we believed there was a greater than 50 percent likelihood that Tuscarora and North Baja’s estimated fair value exceeded their carrying value. As a result, at December 31, 2019, we did not identify an impairment on the $71 million of goodwill related to the Tuscarora ($23 million) and North Baja ($48 million) reporting units. On March 11, 2020, the World Health Organization (WHO) declared COVID-19 a global pandemic. While there are continuing concerns around the decline in energy demand related to the pandemic, we believe the current state of the macroeconomic environment does not represent a permanent shift. However, it is still difficult to predict with any certainty the severity of the impact of these events or how long any disruptions are likely to continue. The following additional factors were considered in our analysis specific to the Partnership's Tuscarora and North Baja reporting units: •the long-term natural gas price futures relevant to gas transported on Tuscarora and North Baja do not reflect material differences from what was forecast in 2019; •at least 90 percent of Tuscarora's and North Baja's revenue is tied to long-term take-or-pay, fixed-price contracts which have a low correlation to short-term changes in demand; •Tuscarora and North Baja have not experienced any material customer defaults to date and have significant collateral in support of their contracts; •multiples and discount rate assumptions used in our quantitative model are reflective of the long-term outlook for Tuscarora and North Baja, in line with their underlying asset lives, versus the shorter-term nature of the current situation; •Tuscarora's expansion project, Tuscarora XPress, is materially on track, and we do not anticipate any significant changes in outlook or delay or inability to proceed due to financing requirements; and •Tuscarora and North Baja's businesses are broadly considered essential in the United States given the important role their infrastructures play in delivering energy to the market areas they serve. As a result of these factors, we concluded during our first quarter 2020 analysis that there was a greater than 50 percent likelihood that both Tuscarora’s and North Baja’s estimated fair values would continue to exceed their carrying values. Therefore, no impairment exists on our goodwill. While the issues described above persist in the current quarter, we continue to believe these conditions remain temporary and are not aware of any other conditions or triggering events in the second quarter that would require us to change the conclusion reached during the first quarter. Adverse changes to our key considerations could,however, result in future impairments on our goodwill.

EQUITY INVESTMENTS

6 Months Ended

Jun. 30, 2020

Equity Method Investments and Joint Ventures [Abstract]

EQUITY INVESTMENTS

The Partnership has equity interests in Northern Border, Great Lakes and Iroquois. The pipeline systems owned by these entities are regulated by the Federal Energy Regulatory Commission (FERC). The Northern Border and Great Lakes pipeline systems are operated by subsidiaries of TC Energy. The Iroquois pipeline system is operated by Iroquois Pipeline Operating Company, a wholly owned subsidiary of Iroquois. The Partnership uses the equity method of accounting for its interests in its equity investees.


	Ownership	Equity Earnings							Equity Investments
	Interest at	Three months ended			Six months ended
(unaudited)	June 30,	June 30,			June 30,			June 30,	December 31,
(millions of dollars)	2020	2020	2019	2020	2019	2020	2019
Northern Border	50.00%	13	14	35	35	412	422
Great Lakes	46.45%	9	9	29	29	489	491
Iroquois	49.34%	7	7	20	20	181	185
		29	30	84	84	1,082	1,098

Distributions from Equity Investments

Distributions received from equity investments in the three and six months ended June 30, 2020 totaled $49 million and $120 million, respectively (June 30, 2019 - $108 million and $167 million, respectively ).

During the six months ended June 30, 2020, $5 million of the total $120 million distributions received from equity investments (June 30, 2019 - $55 million), was considered return of capital and included in "Investing Activities" in the Partnership’s consolidated statement of cash flows. The return of capital was related to our investment in Iroquois (see further discussion below).

Northern Border

During the three and six months ended June 30, 2020, the Partnership received distributions from Northern Border amounting to $18 million and $45 million, respectively (June 30, 2019 - $72 million and $100 million, respectively).

The Partnership did not have undistributed earnings from Northern Border for the three and six months ended June 30, 2020 and 2019.

The summarized financial information provided to us by Northern Border is as follows:


(unaudited)
(millions of dollars)	June 30, 2020	December 31, 2019
ASSETS
Cash and cash equivalents	17	21
Other current assets	38	37
Property, plant and equipment, net	978	989
Other assets	12	12
	1,045	1,059
LIABILITIES AND PARTNERS’ EQUITY
Current liabilities	42	42
Deferred credits and other	41	39
Long-term debt, net ^(a)	369	364
Partners’ equity
Partners’ capital	594	615
Accumulated other comprehensive loss	(1)	(1)
	1,045	1,059


	Three months ended			Six months ended
(unaudited)	June 30,			June 30,
(millions of dollars)	2020	2019	2020	2019
Transmission revenues	66	67	149	148
Operating expenses	(19)	(20)	(39)	(40)
Depreciation	(16)	(16)	(31)	(31)
Financial charges and other	(5)	(4)	(9)	(8)
Net income	26	27	70	69

^(a) No current maturities as of June 30, 2020 and December 31, 2019. At June 30, 2020, Northern Border was in compliance with all its financial covenants.

Great Lakes, a variable interest entity

The Partnership is considered to have a variable interest in Great Lakes, which is accounted for as an equity investment as we are not its primary beneficiary. A variable interest entity is a legal entity that either does not have sufficient equity at risk to finance its activities without additional subordinated financial support, is structured such that equity investors lack the ability to make significant decisions relating to the entity’s operations through voting rights or do not substantively participate in the gains or losses of the entity.

The Partnership made an equity contribution to Great Lakes of $5 million during the six months ended June 30, 2020 (June 30, 2019 - $5 million). This amount represents the Partnership’s 46.45 percent share of an $11 million cash call from Great Lakes to make a scheduled debt repayment.

During the three and six months ended June 30, 2020, the Partnership received distributions from Great Lakes amounting to $21 million and $37 million, respectively (June 30, 2019 - $23 million and $39 million, respectively).

The Partnership did not have undistributed earnings from Great Lakes for the three and six months ended June 30, 2020 and 2019.

The summarized financial information provided to us by Great Lakes is as follows:


(unaudited)
(millions of dollars)	June 30, 2020	December 31, 2019
ASSETS
Current assets	58	72
Property, plant and equipment, net	684	685
	742	757
LIABILITIES AND PARTNERS’ EQUITY
Current liabilities	34	33
Net long-term debt, including current maturities^(a)	208	219
Other long term liabilities	7	6
Partners’ equity	493	499
	742	757


	Three months ended			Six months ended
(unaudited)	June 30,			June 30,
(millions of dollars)	2020	2019	2020	2019
Transmission revenues	50	51	122	123
Operating expenses	(19)	(19)	(35)	(35)
Depreciation	(8)	(8)	(16)	(16)
Financial charges and other	(3)	(5)	(7)	(9)
Net income	20	19	64	63

^(a) Includes current maturities of $31 million as of June 30, 2020 (December 31, 2019 - $21 million). At June 30, 2020, Great Lakes was in compliance with all its financial covenants.

Iroquois

During the three and six months ended June 30, 2020, the Partnership received distributions from Iroquois amounting to $10 million and $38 million, respectively (June 30, 2019 - $14 million and $28 million, respectively), which includes the Partnership’s 49.34 percent share of the Iroquois unrestricted cash distribution amounting to approximately nil and $5.2 million, respectively. (June 30, 2019 - $2.6 million and $5.2 million, respectively). The unrestricted cash did not represent a distribution of Iroquois’ cash from operations during the period and therefore it was reported as a return of investment in the Partnership’s consolidated statement of cash flows.

The Partnership did not have undistributed earnings from Iroquois for the three and six months ended June 30, 2020 and 2019.

The summarized financial information provided to us by Iroquois is as follows:


(unaudited)
(millions of dollars)	June 30, 2020	December 31, 2019
ASSETS
Cash and cash equivalents	35	43
Other current assets	72	36
Property, plant and equipment, net	513	570
Other assets	19	16
	639	665
LIABILITIES AND PARTNERS’ EQUITY
Current liabilities	14	34
Long-term debt, net^(a)	316	317
Other non-current liabilities	22	20
Partners’ equity	287	294
	639	665


	Three months ended			Six months ended
(unaudited)	June 30,			June 30,
(millions of dollars)	2020	2019	2020	2019
Transmission revenues	41	40	93	92
Operating expenses	(14)	(13)	(29)	(28)
Depreciation	(7)	(8)	(15)	(15)
Financial charges and other	(6)	(4)	(9)	(7)
Net income	14	15	40	42

^(a) Includes current maturities of $4 million as of June 30, 2020 (December 31, 2019 - $3 million). At June 30, 2020, Iroquois was in compliance with all of its financial covenants.

REVENUES	6 Months Ended
REVENUES	Jun. 30, 2020
Revenue from Contract with Customer [Abstract]
REVENUES	REVENUES Disaggregation of Revenues For the three and six months ended June 30, 2020 and 2019, effectively all of the Partnership’s revenues were from capacity arrangements and transportation contracts with customers as discussed in more detail below. Capacity Arrangements and Transportation Contracts The Partnership’s performance obligations in its contracts with customers consist primarily of capacity arrangements and natural gas transportation contracts. The Partnership’s revenues are generated from contractual arrangements for committed capacity and from transportation of natural gas which are treated as a bundled performance obligation. Revenues earned from firm contracted capacity arrangements are recognized ratably over the term of the contract regardless of the amount of natural gas that is transported. Transportation revenues for interruptible or volumetric-based services are recognized when the service is performed. The Partnership has elected to utilize the practical expedient of recognizing revenue as invoiced. The Partnership's pipeline systems are subject to FERC regulations and, as a result, a portion of revenues collected may be subject to refund if invoiced during an interim period when a rate proceeding is ongoing. Allowances for these potential refunds are recognized using management's best estimate based on the facts and circumstances of the proceeding. Any allowances that are recognized during the proceeding process are refunded or retained, as applicable, at the time a regulatory decision becomes final. As of June 30, 2020, the Partnership does not have any outstanding refund obligations related to any rate proceedings. Revenues are invoiced and paid on a monthly basis. The Partnership’s pipeline systems do not take ownership of the natural gas that is transported for customers. Revenues from contracts with customers are recognized net of any taxes collected from customers, which are subsequently remitted to governmental authorities. Contract Balances All of the Partnership’s contract balances pertain to receivables from contracts with customers amounting to $32 million at June 30, 2020 (December 31, 2019 - $37 million) and are recorded as trade accounts receivable and reported as "Accounts receivable and other" in the Partnership’s consolidated balance sheet (Refer to Note 14, "Accounts Receivable and Other"). Additionally, our accounts receivable represent the Partnership’s unconditional right to consideration for services completed which includes billed and unbilled accounts. Right to invoice practical expedient In the application of the right to invoice practical expedient, the Partnership’s revenues from regulated capacity arrangements are recognized based on rates specified in the contract. Therefore, the amount invoiced, which includes the capacity contracted and variable volume of natural gas transported, corresponds directly to the value the customer received. These revenues are recognized on a monthly basis once the Partnership’s performance obligation to provide capacity has been satisfied.

DEBT AND CREDIT FACILITIES

6 Months Ended

Jun. 30, 2020

Debt Disclosure [Abstract]

DEBT AND CREDIT FACILITIES


(unaudited) (millions of dollars)	June 30, 2020	Weighted Average Interest Rate for the Six Months Ended June 30, 2020		December 31, 2019	Weighted Average Interest Rate for the Year Ended December 31, 2019
TC PipeLines, LP
Senior Credit Facility due 2021	—	—		—	—
2013 Term Loan Facility due 2022	450	2.34%		450	3.52%
4.65% Unsecured Senior Notes due 2021	350	4.65%	^(a)	350	4.65%	^(a)
4.375% Unsecured Senior Notes due 2025	350	4.375%	^(a)	350	4.375%	^(a)
3.90% Unsecured Senior Notes due 2027	500	3.90%	^(a)	500	3.90%	^(a)

GTN
3.12% Series A Senior Notes due 2030	175	3.12%	^(a)	—	—
5.29% Unsecured Senior Notes due 2020	—	—	^(a)	100	5.29%	^(a)
5.69% Unsecured Senior Notes due 2035	150	5.69%	^(a)	150	5.69%	^(a)


PNGTS
Revolving Credit Facility due 2023	71	2.36%		39	3.47%

Tuscarora
Unsecured Term Loan due 2021	23	2.22%		23	3.39%

North Baja
Unsecured Term Loan due 2021	50	2.17%		50	3.34%
	2,119			2,012
Less: unamortized debt issuance costs and debt discount	7			9
Less: current portion ^(b)	350			123
	1,762			1,880

^(a) Fixed interest rate

^(b) At June 30, 2020, this amount included TC PipeLines, LP's $350 million 4.65% Unsecured Senior Notes due in June 2021. At December 31, 2019, this amount included GTN's $100 million 5.29% Unsecured Senior Notes due in June 2020 and Tuscarora's $23 million Unsecured Term Loan that was due in August 2020.

TC PipeLines, LP

The Partnership’s senior facility under revolving credit agreement as amended and restated, dated September 29, 2017 (Senior Credit Facility) consists of a $500 million senior revolving credit facility with a banking syndicate, maturing November 10, 2021. In March 2019, the Partnership repaid all amounts outstanding under its Senior Credit Facility and there was no outstanding balance at either June 30, 2020 or December 31, 2019.

As of June 30, 2020, the variable interest rate exposure related to the term loan facility under a term loan agreement, as amended, dated September 29, 2017 (2013 Term Loan Facility) was hedged using interest rate swaps at an average rate of 3.26 percent (December 31, 2019 - 3.26 percent). Prior to hedging activities, the London Interbank Offered Rate based (LIBOR) interest rate on the 2013 Term Loan Facility was 1.42 percent at June 30, 2020 (December 31, 2019 - 2.94 percent).

The Senior Credit Facility and the 2013 Term Loan Facility require the Partnership to maintain a debt to adjusted cash flow leverage ratio of no greater than 5.00 to 1.00 for each fiscal quarter, except for the fiscal quarter and the two following fiscal quarters in which one or more acquisitions have been executed, in which case the leverage ratio is to be no greater than 5.50 to 1.00. The leverage ratio was 3.90 to 1.00 as of June 30, 2020.

GTN

On June 1, 2020, GTN’s $100 million 5.29% Unsecured Senior Notes became due and were refinanced through a Note Purchase and Private Shelf Agreement whereby GTN issued $175 million of 10-year Series A Senior Notes with a coupon of 3.12% per annum and entered into a 3-year private shelf agreement for an additional $75 million of Senior Notes (Private Shelf Facility). The new Series A Senior Notes do not require any principal payments until maturity on June 1, 2030. Proceeds from the Series A Senior Note issuance were used to repay the outstanding balance of the 5.29% Unsecured Senior Notes and the remaining proceeds will be used to fund the GTN XPress capital expenditures for the balance of 2020. GTN expects to draw the remaining $75 million available under the Private Shelf Facility by the end of 2023, the estimated completion date of GTN XPress. The Private Shelf Agreement and the Unsecured Senior Notes contain a covenant that limits total debt to no greater than 65 percent and 70 percent of GTN’s total capitalization, respectively. GTN’s total debt to total capitalization ratio at June 30, 2020 was 44.5 percent.

PNGTS

PNGTS’ $125 million Revolving Credit Facility requires PNGTS to maintain a leverage ratio of no greater than 5.00 to 1.00. The leverage ratio was 1.15 to 1.00 as of June 30, 2020. During the six months ended June 30, 2020, PNGTS borrowed an additional $32 million on its Revolving Credit Facility to fund its expansion projects.

The LIBOR-based interest rate applicable to PNGTS’s Revolving Credit Facility was 1.42 percent at June 30, 2020 (December 31, 2019 - 2.99 percent).

Tuscarora

Tuscarora’s $23 million variable rate Unsecured Term Loan due August 21, 2020 (Unsecured Term Loan) contains a covenant that requires Tuscarora to maintain a debt service coverage ratio (cash available from operations divided by the sum of interest expense and principal payments) of greater than or equal to 3.00 to 1.00. As of June 30, 2020, the ratio was 9.14 to 1.00.

The LIBOR-based interest rate applicable to Tuscarora’s Unsecured Term Loan Facility was 1.30 percent at June 30, 2020 (December 31, 2019 - 2.82 percent).

On July 23, 2020, Tuscarora's Unsecured Term Loan was amended to extend the maturity date to August 20, 2021 under generally the same terms.

North Baja

North Baja’s $50 million Term Loan Facility contains a covenant that limits total debt to no greater than 70 percent of North Baja’s total capitalization. North Baja’s total debt to total capitalization ratio at June 30, 2020 was 40.50 percent.

The LIBOR-based interest rate applicable to North Baja’s Term Loan Facility was 1.25 percent at June 30, 2020 (December 31, 2019 - 2.77 percent).

Partnership

At June 30, 2020, the Partnership was in compliance with all terms and conditions including its financial covenants and its other covenants including restrictions on entering into mergers, consolidations and sales of assets, granting of liens, material amendments to the Fourth Amended and Restated Agreement of Limited Partnership, as amended to date (Partnership Agreement), incurring additional debt and distributions to unitholders.

The principal repayments required of the Partnership on its debt are as follows:


(unaudited)
(millions of dollars)			Principal Payments
2020			—
2021			423
2022			450
2023			71
2024			—
Thereafter			1,175
			2,119

PARTNERS' EQUITY	6 Months Ended
PARTNERS' EQUITY	Jun. 30, 2020
Partners' Capital Notes [Abstract]
PARTNERS' EQUITY	PARTNERS’ EQUITY Class B units issued to TC Energy The Class B units entitle TC Energy to an annual distribution based on 30 percent of GTN’s annual distributions as follows: (i) 100 percent of distributions above $20 million through March 31, 2020 and (ii) 25 percent of distributions above $20 million thereafter, which equates to 43.75 percent of distributions above $20 million for the year ending December 31, 2020 (Class B Distribution). Additionally, the Class B Distribution will be further reduced by 35 percent, which is equivalent to the percentage by which distributions payable to the common units were reduced in 2018 (Class B Reduction). The Class B Reduction was implemented during the first quarter of 2018 following the Partnership’s common unit distribution reduction of 35 percent. The Class B Reduction will continue to apply to any particular calendar year until distributions payable in respect of common units for such calendar year equal or exceed $3.94 per common unit. For the year ending December 31, 2020, the Class B units’ equity account will be increased by the Class B Distribution, less the Class B Reduction, until such amount is declared for distribution and paid in the first quarter of 2021. During the six months ended June 30, 2020, the 2020 annual Class B Distribution threshold was not exceeded. For the year ended December 31, 2019, the Class B Distribution was $8 million and was declared and paid in the first quarter of 2020.

NET INCOME PER COMMON UNIT

6 Months Ended

Jun. 30, 2020

Earnings Per Share [Abstract]

NET INCOME PER COMMON UNIT

Net income per common unit is computed by dividing net income attributable to controlling interests, after deduction of amounts attributable to TC PipeLines GP., Inc. (General Partner) and Class B units, by the weighted average number of common units outstanding.

The amount allocable to the General Partner equals an amount based upon the General Partner’s two percent general partner interest, plus an amount equal to incentive distributions. Incentive distributions are paid to the General Partner if quarterly cash distributions on the common units exceed levels specified in the Partnership Agreement.

The amount allocable to the Class B units in 2020 will equal 30 percent of GTN’s distributable cash flow during the year ending December 31, 2020 less $20 million, the residual of which is further multiplied by 43.75 percent. This amount is further reduced by the estimated Class B Reduction for 2020, an approximately 35 percent reduction applied to the estimated annual Class B Distribution (December 31, 2019 - $20 million less Class B Reduction). During the three and six months ended June 30, 2020 and 2019, no amounts were allocated to the Class B units as the annual threshold was not exceeded.

Net income per common unit was determined as follows:


(unaudited)	Three months ended June 30,			Six months ended June 30,
(millions of dollars, except per common unit amounts)	2020	2019	2020	2019
Net income attributable to controlling interests	57	55	145	148


Net income attributable to the General Partner	(1)	(1)	(3)	(3)
Net income attributable to common units	56	54	142	145
Weighted average common units outstanding (millions) — basic and diluted	71.3	71.3	71.3	71.3
Net income per common unit — basic and diluted	$0.78	$0.75	$1.99	$2.03

CASH DISTRIBUTIONS PAID TO COMMON UNITS	6 Months Ended
CASH DISTRIBUTIONS PAID TO COMMON UNITS	Jun. 30, 2020
Partners' Capital Account, Distributions [Abstract]
CASH DISTRIBUTIONS PAID TO COMMON UNITS	CASH DISTRIBUTIONS PAID TO COMMON UNITSDuring both the three and six months ended June 30, 2020 and 2019, the Partnership distributed $0.65 and $1.30 per common unit, respectively for a total distribution of $47 million and $95 million, respectively. The total distribution paid includes our General Partner's share for its two percent general partner interest for both the three and six months ended June 30, 2020 and 2019 totaling $1 million and $2 million,respectively. The General Partner did not receive any distributions in respect of its Incentive Distribution Rights (IDRs) in either of the three or six months ended June 30, 2020 and 2019.

CHANGE IN OPERATING WORKING CAPITAL

6 Months Ended

Jun. 30, 2020

Supplemental Cash Flow Elements [Abstract]

CHANGE IN OPERATING WORKING CAPITAL


(unaudited)	Six months ended June 30,
(millions of dollars)	2020	2019
Change in accounts receivable and other ^(a)	6	15
Change in inventories	—	(1)
Change in other current assets	3	3
Change in accounts payable and accrued liabilities ^(a)	—	(12)
Change in accounts payable to affiliates	(2)	—
Change in accrued interest	(1)	(1)
Change in operating working capital	6	4

^(a) Excludes certain non-cash items primarily related to capital accruals and credits.

RELATED PARTY TRANSACTIONS

6 Months Ended

Jun. 30, 2020

RELATED PARTY TRANSACTIONS

The Partnership does not have any employees. The management and operating functions are provided by the General Partner. The General Partner does not receive a management fee in connection with its management of the Partnership. The Partnership reimburses the General Partner for all costs of services provided, including the costs of employee, officer and director compensation and benefits, and all other expenses necessary or appropriate to conduct the business of, and allocable to, the Partnership. Such costs include (i) overhead costs (such as office space and equipment) and (ii) out-of-pocket expenses related to the provision of such services. The Partnership Agreement provides that the General Partner will determine the costs that are allocable to the Partnership in any reasonable manner determined by the General Partner in its sole discretion. For both the three and six months ended June 30, 2020 and 2019, total costs charged to the Partnership by the General Partner were $1 million and $2 million, respectively.

As operator of our pipelines, except Iroquois and a certain portion of the PNGTS facilities, TC Energy’s subsidiaries provide capital and operating services to our pipeline systems. TC Energy’s subsidiaries incur costs on behalf of our pipeline systems, including, but not limited to, employee salary and benefit costs, and property and liability insurance costs. Iroquois does not receive any capital and operating services from TC Energy (Refer to Note 5, "Equity Investments").

Capital and operating costs charged to our pipeline systems, except for Iroquois, for the three and six months ended June 30, 2020 and 2019 by TC Energy’s subsidiaries and amounts payable to TC Energy’s subsidiaries at June 30, 2020 and December 31, 2019 are summarized in the following tables:


	Three months ended			Six months ended
(unaudited)	June 30,			June 30,
(millions of dollars)	2020	2019	2020	2019
Capital and operating costs charged by TC Energy’s subsidiaries to:
Great Lakes^(a)	11	12	22	23
Northern Border^(a)	10	10	20	19
GTN	12	11	24	21
Bison	1	—	1	1
North Baja	1	2	2	3
Tuscarora	1	1	2	2
PNGTS^(a)	1	1	3	3
Impact on the Partnership’s income ^(b):
Great Lakes	5	5	9	10
Northern Border	4	5	8	9
GTN	7	8	15	16
Bison	1	—	1	1
North Baja	1	1	2	2
Tuscarora	1	1	2	2
PNGTS	1	1	2	2


(unaudited)
(millions of dollars)	June 30, 2020	December 31, 2019
Net amounts payable to TC Energy’s subsidiaries are as follows:
Great Lakes^(a)	4	5
Northern Border^(a)	4	4
GTN	4	5
Bison	—	—
North Baja	—	1
Tuscarora	—	—
PNGTS^(a)	1	1

^(a)Represents 100 percent of the costs.

^(b)Represents the Partnership's proportionate share based ownership percentage of these pipelines.

Great Lakes

Great Lakes earns significant transportation revenues from TC Energy and its affiliates, some of which are provided at discounted rates and some at maximum recourse rates. For the three and six months ended June 30, 2020, Great Lakes earned 75 percent and 74 percent, respectively of its transportation revenues from TC Energy and its affiliates (June 30, 2019 - 73 percent for both periods).

At June 30, 2020, $13 million was included in Great Lakes’ receivables with regard to the transportation contracts with TC Energy and its affiliates (December 31, 2019 - $19 million).

In 2018, Great Lakes executed long-term transportation capacity contracts with its affiliate, ANR Pipeline Company (ANR) in anticipation of specific possible future needs. The original total contract value of these contracts was approximately $1.3 billion over a 15-year period. These contracts were subject to certain conditions and provisions, including a reduction option up to the full contract quantity if exercised up to a certain date. During the first quarter of 2020, several amendments were made to these contracts and ANR exercised the right to terminate a significant portion of the contracts amounting to approximately $1.1 billion. The remaining maximum rate contract, which has a total capacity of approximately 168,000 Dth/Day and total contract value of $182 million over a term of 20 years, is expected to begin in late 2022. This contract, which has a full quantity reduction option at any time before October 1, 2022, is dependent on ANR being able to secure the required regulatory approvals and other requirements of the project associated with these volumes. Any remaining unsubscribed capacity on Great Lakes will be available for contracting in response to developing marketing conditions.

PNGTS

In connection with the Portland XPress expansion project (PXP), which was designed to be phased in over a three year time period, PNGTS has entered into an arrangement with affiliates regarding the construction of certain facilities on their systems that are required to fulfill future contracts on the PNGTS system. PXP Phases I and II were placed into service on November 1, 2018 and November 1, 2019, respectively. Phase III is anticipated to be in service on November 1, 2020. In the event the expansions are terminated prior to their in-service dates, PNGTS will be required to reimburse its affiliates for any costs incurred related to the development of their facilities. As a result of placing the TC Energy facilities associated with the Phase II volumes in service, PNGTS' obligation for Phase II costs has been extinguished. PNGTS' remaining potential obligation to reimburse costs incurred by TC Energy in relation to Phase III, which was approximately $3.8 million at June 30, 2020 and estimated to be approximately $9.1 million by November 1, 2020, will be terminated when Phase III goes into service.

FAIR VALUE MEASUREMENTS	6 Months Ended
FAIR VALUE MEASUREMENTS	Jun. 30, 2020
Fair Value Disclosures [Abstract]
FAIR VALUE MEASUREMENTS	FAIR VALUE MEASUREMENTS (a) Fair Value Hierarchy Under Accounting Standards Codification (ASC) 820, Fair Value Measurements and Disclosures, fair value measurements are characterized in one of three levels based upon the inputs used to arrive at the measurement. The three levels of the fair value hierarchy are as follows: •Level 1 inputs are quoted prices (unadjusted) in active markets for identical assets or liabilities that we have the ability to access at the measurement date. •Level 2 inputs are inputs other than quoted prices included within Level 1 that are observable for the asset or liability, either directly or indirectly. •Level 3 inputs are unobservable inputs for the asset or liability. When appropriate, valuations are adjusted for various factors including credit considerations. Such adjustments are generally based on available market evidence. In the absence of such evidence, management’s best estimate is used. (b) Fair Value of Financial Instruments The carrying value of "cash and cash equivalents,""accounts receivable and other,""accounts payable and accrued liabilities,""accounts payable to affiliates" and "accrued interest" approximate their fair values because of the short maturity or duration of these instruments, or because the instruments bear a variable rate of interest or a rate that approximates current rates. The fair value of the Partnership’s debt is estimated by discounting the future cash flows of each instrument at estimated current borrowing rates. The fair value of interest rate derivatives is calculated using the income approach, which uses period-end market rates and applies a discounted cash flow valuation model. The Partnership has classified the fair value of natural gas imbalances as a Level 2 of the fair value hierarchy for fair value disclosure purposes, as the valuation approach includes quoted prices in the market index and observable volumes for the imbalance. Long-term debt is recorded at amortized cost and classified as Level 2 of the fair value hierarchy for fair value disclosure purposes. Interest rate derivative assets and liabilities are classified as Level 2 for all periods presented where the fair value is determined by using valuation techniques that refer to observable market data or estimated market prices. The estimated fair value of the Partnership’s debt as at June 30, 2020 and December 31, 2019 was $2,223 million and $2,111 million, respectively. Market risk is the risk that changes in market interest rates may result in fluctuations in the fair values or cash flows of financial instruments. The Partnership’s floating rate debt is subject to LIBOR benchmark interest rate risk. The Partnership uses derivatives to manage its exposure to interest rate risk. We regularly assess the impact of interest rate fluctuations on future cash flows and evaluate hedging opportunities to mitigate our interest rate risk. The Partnership’s interest rate swaps mature on October 2, 2022. The interest rate swaps were structured such that the cash flows of the derivative instruments match the variable rate of interest on the 2013 Term Loan Facility. The fixed weighted average interest rate on these instruments is 3.26 percent. At June 30, 2020, the fair value of the interest rate swaps accounted for as cash flow hedges was a liability of $19 million (both on a gross and net basis) (December 31, 2019 - liability of $6 million), the net change of which is recognized in other comprehensive income. For both the three and six months ended June 30, 2020, the net realized loss related to the interest rate swaps was $2 million and was included in "financial charges and other" (June 30, 2019 - gain of nil and gain of $1 million, respectively) (Refer to Note 15, "Financial Charges and Other'). The Partnership has no master netting agreements; however, it has derivative contracts containing provisions with rights of offset. The Partnership has elected to present the fair value of derivative instruments with the right to offset on a gross basis in the consolidated balance sheet. Had the Partnership elected to present these instruments on a net basis, there would be no effect on the consolidated balance sheet as of June 30, 2020 and December 31, 2019.

ACCOUNTS RECEIVABLE AND OTHER

6 Months Ended

Jun. 30, 2020

Receivables [Abstract]

ACCOUNTS RECEIVABLE AND OTHER


(unaudited)
(millions of dollars)	June 30, 2020	December 31, 2019
Trade accounts receivable, net of allowance of nil	32	37

Other	2	6
	34	43

FINANCIAL CHARGES AND OTHER

6 Months Ended

Jun. 30, 2020

Other Expense, Nonoperating [Abstract]

FINANCIAL CHARGES AND OTHER


	Three months ended			Six months ended
(unaudited)	June 30,			June 30,
(millions of dollars)	2020	2019	2020	2019
Interest expense^(a)	20	22	40	45
Net realized loss (gain) related to the interest rate swaps	2	—	2	(1)
Other income	(4)	(1)	(5)	(1)
	18	21	37	43

^(a)Includes amortization of debt issuance costs and discount costs.

SUBSEQUENT EVENTS	6 Months Ended
SUBSEQUENT EVENTS	Jun. 30, 2020
Subsequent Events [Abstract]
SUBSEQUENT EVENTS	SUBSEQUENT EVENTS Management of the Partnership has reviewed subsequent events through August 5, 2020, the date the consolidated financial statements were issued, and concluded there were no events or transactions during this period that would require recognition or disclosure in the consolidated financial statements other than what is disclosed here and/or those already disclosed in the preceding notes. On July 23, 2020, the board of directors of the General Partner declared the Partnership’s second quarter 2020 cash distribution in the amount of $0.65 per common unit payable on August 14, 2020 to unitholders of record as of August 3, 2020. The declared distribution totaled $47 million and is payable in the following manner: $46 million to common unitholders (including $4 million to the General Partner as a holder of 5,797,106 common units and $7 million to another subsidiary of TC Energy as holder of 11,287,725 common units) and $1 million to the General Partner for its two percent general partner interest. The General Partner did not receive any distributions in respect of its IDRs for the second quarter of 2020. Northern Border declared its June 2020 distribution of $11 million on July 9 2020, of which the Partnership received its 50 percent share or $5 million on July 31, 2020. Great Lakes declared its second quarter 2020 distribution of $24 million on July 15, 2020, of which the Partnership received its 46.45 percent share or $11 million on July 31, 2020. Iroquois declared its second quarter 2020 distribution of $21 million on August 4, 2020, of which the Partnership will receive its 49.34 percent share or $10 million on September 29, 2020. PNGTS declared its second quarter 2020 distribution of $14 million on July 8, 2020, of which $5 million was paid to its non-controlling interest owner on July 31, 2020. On August 1, 2020, GTN, Great Lakes, Tuscarora and North Baja entered into a purchase agreement with a TC Energy affiliate to purchase an internally developed customer-facing commercial natural gas transmission IT application that maintains and manages customer contracts, natural gas capacity release, customer nominations, metering and billings. The total value of the transaction was $51 million and the Partnership's proportionate share of the cost was $38 million. Prior to the transaction close, GTN, Great Lakes, Tuscarora and North Baja paid the affiliate for the use of this system and the costs are included in the "Impact on Partnership's income" tabular summary under Note 12.

SIGNIFICANT ACCOUNTING POLICIES (Policies)	6 Months Ended
SIGNIFICANT ACCOUNTING POLICIES (Policies)	Jun. 30, 2020
Accounting Policies [Abstract]
Basis of Accounting	The accompanying consolidated financial statements and related notes have been prepared in accordance with United States generally accepted accounting principles (GAAP) and amounts are stated in U.S. dollars. The results of operations for the three and six months ended June 30, 2020 and 2019 are not necessarily indicative of the results that may be expected for the full fiscal year. The accompanying consolidated financial statements should be read in conjunction with the audited consolidated financial statements and notes thereto for the year ended December 31, 2019 included in our 2019 Annual Report. That report contains a more comprehensive summary of the Partnership’s significant accounting policies. In the opinion of management, the accompanying consolidated financial statements contain all of the appropriate adjustments, which are normally recurring adjustments unless otherwise noted, and considered necessary to present fairly the financial position of the Partnership, the results of operations and cash flows for the respective periods. Our significant accounting policies are consistent with those disclosed in our 2019 Annual Report, except those that became effective in 2020 as described in full under Note 3, "Accounting Pronouncements."
Basis of Presentation - Consolidation and equity method of accounting	Basis of Presentation The Partnership consolidates its interests in entities over which it is able to exercise control. To the extent there are interests owned by other parties, these interests are included as non-controlling interests. The Partnership uses the equity method of accounting for its investments in entities over which it is able to exercise significant influence.
Basis of Presentation - Transactions between entities under common control	U.S. federal and certain state income taxes are the responsibility of the limited partners and are not reflected in these consolidated financial statements. The tax effect of the Partnership's activities accrues to its limited partners. The Partnership’s taxable income or loss, which may vary substantially from the net income or loss reported in the consolidated statement of operations, is includable in the U.S. federal income tax returns of each partner. In instances where the Partnership’s consolidated entities are subject to state income taxes, the asset-liability method is used to account for taxes. This method requires recognition of deferred tax assets and liabilities for future tax consequences attributable to the differences between the financial statement carrying amount of existing assets and liabilities and their respective tax bases. Deferred tax assets and liabilities are classified as non-current on our consolidated balance sheets.
Use of Estimates	Use of Estimates The preparation of financial statements requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities, if any, as of the date of the financial statements, and the reported amounts of revenues and expenses during the reporting period. Although management believes these estimates are reasonable, actual results could differ from these estimates.
Changes in Accounting Policies	Changes in Accounting Policies effective January 1, 2020 Measurement of credit losses on financial instruments In June 2016, the Financial Accounting Standards Board (FASB) issued new guidance that changes how entities measure credit losses for most financial assets and certain other financial instruments that are not measured at fair value through net income (loss). The new guidance amends the impairment model of financial instruments basing it on expected losses rather than incurred losses. These expected credit losses will be recognized as an allowance rather than as a direct write down of the amortized cost basis. The new guidance became effective January 1, 2020 and was applied using a modified retrospective approach. The adoption of this new guidance did not have a material impact on the Partnership’s consolidated financial statements. Consolidation In October 2018, the FASB issued new guidance for determining whether fees paid to decision makers and service providers are variable interests for indirect interests held through related parties under common control. This new guidance became effective January 1, 2020, and was applied on a retrospective basis. The adoption of this new guidance did not have a material impact on the Partnership’s consolidated financial statements. Reference rate reform In March 2020, in response to the expected cessation of LIBOR, the FASB issued new optional guidance that eases the potential burden of accounting for reference rate reform. The new guidance provides optional expedients for contracts and hedging relationships that are affected by reference rate reform, if certain criteria are met. Each of the expedients can be applied as of January 1, 2020 through December 31, 2022. For eligible hedging relationships existing as of January 1, 2020 and prospectively, the Partnership has applied the optional expedient allowing an entity to assume that the hedged forecasted transaction in a cash flow hedge is probable of occurring. As reference rate reform is still an ongoing process, the Partnership will continue to evaluate the timing and potential impact of adoption of other optional expedients when deemed necessary.

EQUITY INVESTMENTS (Tables)

6 Months Ended

Jun. 30, 2020

Equity Method Investments and Joint Ventures [Abstract]

Summary of equity investments


	Ownership	Equity Earnings							Equity Investments
	Interest at	Three months ended			Six months ended
(unaudited)	June 30,	June 30,			June 30,			June 30,	December 31,
(millions of dollars)	2020	2020	2019	2020	2019	2020	2019
Northern Border	50.00%	13	14	35	35	412	422
Great Lakes	46.45%	9	9	29	29	489	491
Iroquois	49.34%	7	7	20	20	181	185
		29	30	84	84	1,082	1,098

The summarized financial information provided to us by Northern Border is as follows:


(unaudited)
(millions of dollars)	June 30, 2020	December 31, 2019
ASSETS
Cash and cash equivalents	17	21
Other current assets	38	37
Property, plant and equipment, net	978	989
Other assets	12	12
	1,045	1,059
LIABILITIES AND PARTNERS’ EQUITY
Current liabilities	42	42
Deferred credits and other	41	39
Long-term debt, net ^(a)	369	364
Partners’ equity
Partners’ capital	594	615
Accumulated other comprehensive loss	(1)	(1)
	1,045	1,059


	Three months ended			Six months ended
(unaudited)	June 30,			June 30,
(millions of dollars)	2020	2019	2020	2019
Transmission revenues	66	67	149	148
Operating expenses	(19)	(20)	(39)	(40)
Depreciation	(16)	(16)	(31)	(31)
Financial charges and other	(5)	(4)	(9)	(8)
Net income	26	27	70	69

^(a) No current maturities as of June 30, 2020 and December 31, 2019. At June 30, 2020, Northern Border was in compliance with all its financial covenants.

The summarized financial information provided to us by Great Lakes is as follows:


(unaudited)
(millions of dollars)	June 30, 2020	December 31, 2019
ASSETS
Current assets	58	72
Property, plant and equipment, net	684	685
	742	757
LIABILITIES AND PARTNERS’ EQUITY
Current liabilities	34	33
Net long-term debt, including current maturities^(a)	208	219
Other long term liabilities	7	6
Partners’ equity	493	499
	742	757


	Three months ended			Six months ended
(unaudited)	June 30,			June 30,
(millions of dollars)	2020	2019	2020	2019
Transmission revenues	50	51	122	123
Operating expenses	(19)	(19)	(35)	(35)
Depreciation	(8)	(8)	(16)	(16)
Financial charges and other	(3)	(5)	(7)	(9)
Net income	20	19	64	63

^(a) Includes current maturities of $31 million as of June 30, 2020 (December 31, 2019 - $21 million). At June 30, 2020, Great Lakes was in compliance with all its financial covenants.

The summarized financial information provided to us by Iroquois is as follows:


(unaudited)
(millions of dollars)	June 30, 2020	December 31, 2019
ASSETS
Cash and cash equivalents	35	43
Other current assets	72	36
Property, plant and equipment, net	513	570
Other assets	19	16
	639	665
LIABILITIES AND PARTNERS’ EQUITY
Current liabilities	14	34
Long-term debt, net^(a)	316	317
Other non-current liabilities	22	20
Partners’ equity	287	294
	639	665


	Three months ended			Six months ended
(unaudited)	June 30,			June 30,
(millions of dollars)	2020	2019	2020	2019
Transmission revenues	41	40	93	92
Operating expenses	(14)	(13)	(29)	(28)
Depreciation	(7)	(8)	(15)	(15)
Financial charges and other	(6)	(4)	(9)	(7)
Net income	14	15	40	42

^(a) Includes current maturities of $4 million as of June 30, 2020 (December 31, 2019 - $3 million). At June 30, 2020, Iroquois was in compliance with all of its financial covenants.

DEBT AND CREDIT FACILITIES (Tables)

6 Months Ended

Jun. 30, 2020

Debt Disclosure [Abstract]

Schedule of debt and credit facilities


(unaudited) (millions of dollars)	June 30, 2020	Weighted Average Interest Rate for the Six Months Ended June 30, 2020		December 31, 2019	Weighted Average Interest Rate for the Year Ended December 31, 2019
TC PipeLines, LP
Senior Credit Facility due 2021	—	—		—	—
2013 Term Loan Facility due 2022	450	2.34%		450	3.52%
4.65% Unsecured Senior Notes due 2021	350	4.65%	^(a)	350	4.65%	^(a)
4.375% Unsecured Senior Notes due 2025	350	4.375%	^(a)	350	4.375%	^(a)
3.90% Unsecured Senior Notes due 2027	500	3.90%	^(a)	500	3.90%	^(a)

GTN
3.12% Series A Senior Notes due 2030	175	3.12%	^(a)	—	—
5.29% Unsecured Senior Notes due 2020	—	—	^(a)	100	5.29%	^(a)
5.69% Unsecured Senior Notes due 2035	150	5.69%	^(a)	150	5.69%	^(a)


PNGTS
Revolving Credit Facility due 2023	71	2.36%		39	3.47%

Tuscarora
Unsecured Term Loan due 2021	23	2.22%		23	3.39%

North Baja
Unsecured Term Loan due 2021	50	2.17%		50	3.34%
	2,119			2,012
Less: unamortized debt issuance costs and debt discount	7			9
Less: current portion ^(b)	350			123
	1,762			1,880

^(a) Fixed interest rate

Schedule of principal repayments required on debt

The principal repayments required of the Partnership on its debt are as follows:


(unaudited)
(millions of dollars)			Principal Payments
2020			—
2021			423
2022			450
2023			71
2024			—
Thereafter			1,175
			2,119

NET INCOME PER COMMON UNIT (Tables)

6 Months Ended

Jun. 30, 2020

Earnings Per Share [Abstract]

Schedule of net income per common unit

Net income per common unit was determined as follows:


(unaudited)	Three months ended June 30,			Six months ended June 30,
(millions of dollars, except per common unit amounts)	2020	2019	2020	2019
Net income attributable to controlling interests	57	55	145	148


Net income attributable to the General Partner	(1)	(1)	(3)	(3)
Net income attributable to common units	56	54	142	145
Weighted average common units outstanding (millions) — basic and diluted	71.3	71.3	71.3	71.3
Net income per common unit — basic and diluted	$0.78	$0.75	$1.99	$2.03

CHANGE IN OPERATING WORKING CAPITAL (Tables)

6 Months Ended

Jun. 30, 2020

Supplemental Cash Flow Elements [Abstract]

Schedule of change in operating working capital


(unaudited)	Six months ended June 30,
(millions of dollars)	2020	2019
Change in accounts receivable and other ^(a)	6	15
Change in inventories	—	(1)
Change in other current assets	3	3
Change in accounts payable and accrued liabilities ^(a)	—	(12)
Change in accounts payable to affiliates	(2)	—
Change in accrued interest	(1)	(1)
Change in operating working capital	6	4

^(a) Excludes certain non-cash items primarily related to capital accruals and credits.

RELATED PARTY TRANSACTIONS (Tables)

6 Months Ended

Jun. 30, 2020

Summary of related party transactions


	Three months ended			Six months ended
(unaudited)	June 30,			June 30,
(millions of dollars)	2020	2019	2020	2019
Capital and operating costs charged by TC Energy’s subsidiaries to:
Great Lakes^(a)	11	12	22	23
Northern Border^(a)	10	10	20	19
GTN	12	11	24	21
Bison	1	—	1	1
North Baja	1	2	2	3
Tuscarora	1	1	2	2
PNGTS^(a)	1	1	3	3
Impact on the Partnership’s income ^(b):
Great Lakes	5	5	9	10
Northern Border	4	5	8	9
GTN	7	8	15	16
Bison	1	—	1	1
North Baja	1	1	2	2
Tuscarora	1	1	2	2
PNGTS	1	1	2	2


(unaudited)
(millions of dollars)	June 30, 2020	December 31, 2019
Net amounts payable to TC Energy’s subsidiaries are as follows:
Great Lakes^(a)	4	5
Northern Border^(a)	4	4
GTN	4	5
Bison	—	—
North Baja	—	1
Tuscarora	—	—
PNGTS^(a)	1	1

^(a)Represents 100 percent of the costs.

^(b)Represents the Partnership's proportionate share based ownership percentage of these pipelines.

ACCOUNTS RECEIVABLE AND OTHER (Tables)

6 Months Ended

Jun. 30, 2020

Receivables [Abstract]

Schedule of accounts receivable and other


(unaudited)
(millions of dollars)	June 30, 2020	December 31, 2019
Trade accounts receivable, net of allowance of nil	32	37

Other	2	6
	34	43

FINANCIAL CHARGES AND OTHER (Tables)

6 Months Ended

Jun. 30, 2020

Other Expense, Nonoperating [Abstract]

Schedule of components of financial charges and other


	Three months ended			Six months ended
(unaudited)	June 30,			June 30,
(millions of dollars)	2020	2019	2020	2019
Interest expense^(a)	20	22	40	45
Net realized loss (gain) related to the interest rate swaps	2	—	2	(1)
Other income	(4)	(1)	(5)	(1)
	18	21	37	43

^(a)Includes amortization of debt issuance costs and discount costs.

GOODWILL (Details) - USD ($) $ in Millions	Jun. 30, 2020	Dec. 31, 2019
Goodwill [Line Items]
Goodwill	$ 71	$ 71
Tuscarora
Goodwill [Line Items]
Goodwill		23
North Baja
Goodwill [Line Items]
Goodwill		$ 48

EQUITY INVESTMENTS - Summary of equity investments (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
Schedule of Equity Method Investments [Line Items]
Equity Earnings	$ 29,000,000	$ 30,000,000	$ 84,000,000	$ 84,000,000
Equity Investments	1,082,000,000		1,082,000,000		$ 1,098,000,000
ASSETS
Cash and cash equivalents	215,000,000		215,000,000		83,000,000
Other current assets	3,000,000		3,000,000		6,000,000
Current assets	262,000,000		262,000,000		156,000,000
Property, plant and equipment, net	1,598,000,000		1,598,000,000		1,528,000,000
TOTAL ASSETS	3,013,000,000		3,013,000,000		2,853,000,000
LIABILITIES AND PARTNERS’ EQUITY
Current liabilities	415,000,000		415,000,000		170,000,000
Other liabilities	43,000,000		43,000,000		36,000,000
Long-term debt, net	2,119,000,000		2,119,000,000
Accumulated other comprehensive income (loss) (AOCI)	(18,000,000)		(18,000,000)		(5,000,000)
Partners’ equity	787,000,000		787,000,000		760,000,000
TOTAL LIABILITIES AND PARTNERS’ EQUITY	3,013,000,000		3,013,000,000		2,853,000,000
NET INCOME
Transmission revenues	95,000,000	93,000,000	196,000,000	206,000,000
Depreciation and amortization	(19,000,000)	(19,000,000)	(39,000,000)	(39,000,000)
Financial charges and other	(18,000,000)	(21,000,000)	(37,000,000)	(43,000,000)
Net income attributable to controlling interests	57,000,000	55,000,000	145,000,000	148,000,000
Current portion of long-term debt	350,000,000		350,000,000		123,000,000
Northern Border
ASSETS
Cash and cash equivalents	17,000,000		17,000,000		21,000,000
Other current assets	38,000,000		38,000,000		37,000,000
Property, plant and equipment, net	978,000,000		978,000,000		989,000,000
Other assets	12,000,000		12,000,000		12,000,000
TOTAL ASSETS	1,045,000,000		1,045,000,000		1,059,000,000
LIABILITIES AND PARTNERS’ EQUITY
Current liabilities	42,000,000		42,000,000		42,000,000
Other liabilities	41,000,000		41,000,000		39,000,000
Long-term debt, net	369,000,000		369,000,000		364,000,000
Partners’ capital	594,000,000		594,000,000		615,000,000
Accumulated other comprehensive income (loss) (AOCI)	(1,000,000)		(1,000,000)		(1,000,000)
TOTAL LIABILITIES AND PARTNERS’ EQUITY	1,045,000,000		1,045,000,000		1,059,000,000
NET INCOME
Transmission revenues	66,000,000	67,000,000	149,000,000	148,000,000
Operating expenses	(19,000,000)	(20,000,000)	(39,000,000)	(40,000,000)
Depreciation and amortization	(16,000,000)	(16,000,000)	(31,000,000)	(31,000,000)
Financial charges and other	(5,000,000)	(4,000,000)	(9,000,000)	(8,000,000)
Net income attributable to controlling interests	26,000,000	27,000,000	70,000,000	69,000,000
Current portion of long-term debt	0		0		0
Great Lakes
ASSETS
Current assets	58,000,000		58,000,000		72,000,000
Property, plant and equipment, net	684,000,000		684,000,000		685,000,000
TOTAL ASSETS	742,000,000		742,000,000		757,000,000
LIABILITIES AND PARTNERS’ EQUITY
Current liabilities	34,000,000		34,000,000		33,000,000
Other liabilities	7,000,000		7,000,000		6,000,000
Long-term debt, net	208,000,000		208,000,000		219,000,000
Partners’ equity	493,000,000		493,000,000		499,000,000
TOTAL LIABILITIES AND PARTNERS’ EQUITY	742,000,000		742,000,000		757,000,000
NET INCOME
Transmission revenues	50,000,000	51,000,000	122,000,000	123,000,000
Operating expenses	(19,000,000)	(19,000,000)	(35,000,000)	(35,000,000)
Depreciation and amortization	(8,000,000)	(8,000,000)	(16,000,000)	(16,000,000)
Financial charges and other	(3,000,000)	(5,000,000)	(7,000,000)	(9,000,000)
Net income attributable to controlling interests	20,000,000	19,000,000	64,000,000	63,000,000
Current portion of long-term debt	31,000,000		31,000,000		21,000,000
Iroquois
ASSETS
Cash and cash equivalents	35,000,000		35,000,000		43,000,000
Other current assets	72,000,000		72,000,000		36,000,000
Property, plant and equipment, net	513,000,000		513,000,000		570,000,000
Other assets	19,000,000		19,000,000		16,000,000
TOTAL ASSETS	639,000,000		639,000,000		665,000,000
LIABILITIES AND PARTNERS’ EQUITY
Current liabilities	14,000,000		14,000,000		34,000,000
Other liabilities	22,000,000		22,000,000		20,000,000
Long-term debt, net	316,000,000		316,000,000		317,000,000
Partners’ equity	287,000,000		287,000,000		294,000,000
TOTAL LIABILITIES AND PARTNERS’ EQUITY	639,000,000		639,000,000		665,000,000
NET INCOME
Transmission revenues	41,000,000	40,000,000	93,000,000	92,000,000
Operating expenses	(14,000,000)	(13,000,000)	(29,000,000)	(28,000,000)
Depreciation and amortization	(7,000,000)	(8,000,000)	(15,000,000)	(15,000,000)
Financial charges and other	(6,000,000)	(4,000,000)	(9,000,000)	(7,000,000)
Net income attributable to controlling interests	14,000,000	15,000,000	40,000,000	42,000,000
Current portion of long-term debt	$ 4,000,000		$ 4,000,000		3,000,000
Northern Border
Schedule of Equity Method Investments [Line Items]
Ownership Interest	50.00%		50.00%
Equity Earnings	$ 13,000,000	$ 14,000,000	$ 35,000,000	$ 35,000,000
Equity Investments	$ 412,000,000		$ 412,000,000		422,000,000
Great Lakes
Schedule of Equity Method Investments [Line Items]
Ownership Interest	46.45%	46.45%	46.45%	46.45%
Equity Earnings	$ 9,000,000	$ 9,000,000	$ 29,000,000	$ 29,000,000
Equity Investments	$ 489,000,000		$ 489,000,000		491,000,000
Iroquois
Schedule of Equity Method Investments [Line Items]
Ownership Interest	49.34%	49.34%	49.34%	49.34%
Equity Earnings	$ 7,000,000	$ 7,000,000	$ 20,000,000	$ 20,000,000
Equity Investments	$ 181,000,000		$ 181,000,000		$ 185,000,000

EQUITY INVESTMENTS - Narrative (Details) - USD ($)	3 Months Ended		6 Months Ended
EQUITY INVESTMENTS - Narrative (Details) - USD ($)	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Schedule of Equity Method Investments [Line Items]
Distributions received from equity investments	$ 49,000,000	$ 108,000,000	$ 120,000,000	$ 167,000,000
Return on investment distribution classified as investing activities			5,000,000	55,000,000
Partnership's share of distributions			115,000,000	112,000,000
Iroquois
Schedule of Equity Method Investments [Line Items]
Return on investment distribution classified as investing activities			$ 5,000,000	$ 5,000,000
Ownership interest	49.34%	49.34%	49.34%	49.34%
Partnership's share of distributions	$ 10,000,000	$ 14,000,000	$ 38,000,000	$ 28,000,000
Northern Border
Schedule of Equity Method Investments [Line Items]
Distributions received from equity investments	$ 18,000,000	72,000,000	45,000,000	100,000,000
Return on investment distribution classified as investing activities			$ 0	50,000,000
Ownership interest	50.00%		50.00%
Great Lakes
Schedule of Equity Method Investments [Line Items]
Distributions received from equity investments	$ 21,000,000	$ 23,000,000	$ 37,000,000	39,000,000
Equity contribution			$ 5,000,000	$ 5,000,000
Ownership interest	46.45%	46.45%	46.45%	46.45%
Iroquois
Schedule of Equity Method Investments [Line Items]
Return on investment distribution classified as investing activities	$ 0	$ 2,600,000	$ 5,200,000	$ 5,200,000
Great Lakes
Schedule of Equity Method Investments [Line Items]
Total cash call issued to fund debt repayment			$ 11,000,000

REVENUES (Details) - USD ($) $ in Millions	Jun. 30, 2020	Dec. 31, 2019
Contract Balances
Receivables from contracts with customers	$ 32	$ 37

DEBT AND CREDIT FACILITIES - Schedule of debt and credit facilities (Details) - USD ($) $ in Millions	6 Months Ended	12 Months Ended
	Jun. 30, 2020	Dec. 31, 2019	Jun. 01, 2020
Debt Instrument [Line Items]
Debt, gross	$ 2,119	$ 2,012
Less: unamortized debt issuance costs and debt discount	7	9
Current portion of long-term debt	350	123
Debt, net	1,762	1,880
Revolving credit facility \| Senior Credit Facility due 2021
Debt Instrument [Line Items]
Debt, gross	$ 0	$ 0
Weighted average interest rate	0.00%	0.00%
Term loan \| 2013 Term Loan Facility due 2022
Debt Instrument [Line Items]
Debt, gross	$ 450	$ 450
Weighted average interest rate	2.34%	3.52%
Unsecured debt \| 4.65% Unsecured Senior Notes due 2021
Debt Instrument [Line Items]
Stated interest rate	4.65%
Debt, gross	$ 350	$ 350
Weighted average interest rate	4.65%	4.65%
Unsecured debt \| 4.375% Unsecured Senior Notes due 2025
Debt Instrument [Line Items]
Stated interest rate	4.375%
Debt, gross	$ 350	$ 350
Weighted average interest rate	4.375%	4.375%
Unsecured debt \| 3.90% Unsecured Senior Notes due 2027
Debt Instrument [Line Items]
Stated interest rate	3.90%
Debt, gross	$ 500	$ 500
Weighted average interest rate	3.90%	3.90%
GTN \| Secured debt \| 3.12% Series A Senior Notes due 2030
Debt Instrument [Line Items]
Stated interest rate	3.12%		3.12%
Debt, gross	$ 175	$ 0
Weighted average interest rate	3.12%	0.00%
GTN \| Unsecured debt \| 5.29% Unsecured Senior Notes due 2020
Debt Instrument [Line Items]
Stated interest rate	5.29%	5.29%	5.29%
Debt, gross	$ 0	$ 100
Weighted average interest rate	0.00%	5.29%
GTN \| Unsecured debt \| 5.69% Unsecured Senior Notes due 2035
Debt Instrument [Line Items]
Stated interest rate	5.69%
Debt, gross	$ 150	$ 150
Weighted average interest rate	5.69%	5.69%
PNGTS \| Secured debt \| Revolving Credit Facility due 2023
Debt Instrument [Line Items]
Debt, gross	$ 71	$ 39
Weighted average interest rate	2.36%	3.47%
Tuscarora \| Unsecured debt \| Unsecured Term Loan due 2021
Debt Instrument [Line Items]
Debt, gross	$ 23	$ 23
Weighted average interest rate	2.22%	3.39%
North Baja \| Unsecured debt \| Unsecured Term Loan due 2021
Debt Instrument [Line Items]
Debt, gross	$ 50	$ 50
Weighted average interest rate	2.17%	3.34%

DEBT AND CREDIT FACILITIES - Narrative (Details) - USD ($)		6 Months Ended
	Jun. 01, 2020	Jun. 30, 2020	Dec. 31, 2019
Credit facilities, short-term loan facility and long-term debt
Debt, gross		$ 2,119,000,000	$ 2,012,000,000
Senior Credit Facility due in 2021 and the Term Loan Facilities due in 2022
Credit facilities, short-term loan facility and long-term debt
Leverage ratio, actual (as a percent)		3.90%
Senior Credit Facility due in 2021 and the Term Loan Facilities due in 2022 \| Each fiscal quarter except fiscal quarter and the two following fiscal quarters in which one or more acquisitions has been executed \| Maximum
Credit facilities, short-term loan facility and long-term debt
Leverage ratio, covenant (as a percent)		5.00%
Senior Credit Facility due in 2021 and the Term Loan Facilities due in 2022 \| Fiscal quarter and the two following fiscal quarters in which one or more acquisitions has been executed \| Maximum
Credit facilities, short-term loan facility and long-term debt
Leverage ratio, covenant (as a percent)		5.50%
Revolving credit facility \| Senior Credit Facility due 2021
Credit facilities, short-term loan facility and long-term debt
Maximum borrowing capacity		$ 500,000,000
Amount outstanding under credit facility		0	0
Debt, gross		0	0
Term loan \| 2013 Term Loan Facility due 2022
Credit facilities, short-term loan facility and long-term debt
Debt, gross		$ 450,000,000	$ 450,000,000
Term loan \| 2013 Term Loan Facility due 2022 \| Interest rate swaps
Credit facilities, short-term loan facility and long-term debt
Weighted average fixed interest rate		3.26%	3.26%
Term loan \| 2013 Term Loan Facility due 2022 \| LIBOR borrowings \| LIBOR
Credit facilities, short-term loan facility and long-term debt
Debt interest rate, at period end		1.42%	2.94%
GTN \| Unsecured debt \| 5.29% Unsecured Senior Notes due 2020
Credit facilities, short-term loan facility and long-term debt
Face amount	$ 100,000,000
Stated interest rate	5.29%	5.29%	5.29%
Debt, gross		$ 0	$ 100,000,000
GTN \| Unsecured debt \| 5.69% Unsecured Senior Notes due 2035
Credit facilities, short-term loan facility and long-term debt
Stated interest rate		5.69%
Percentage of debt to total capitalization, covenant		70.00%
Debt, gross		$ 150,000,000	150,000,000
GTN \| Secured debt \| 3.12% Series A Senior Notes due 2030
Credit facilities, short-term loan facility and long-term debt
Face amount	$ 175,000,000
Stated interest rate	3.12%	3.12%
Debt term	10 years
Debt, gross		$ 175,000,000	0
GTN \| Secured debt \| Private Shelf Facility
Credit facilities, short-term loan facility and long-term debt
Face amount	$ 75,000,000
Debt term	3 years
Percentage of debt to total capitalization, actual		44.50%
GTN \| Secured debt \| Private Shelf Facility \| Maximum
Credit facilities, short-term loan facility and long-term debt
Percentage of debt to total capitalization, covenant		65.00%
PNGTS \| Secured debt \| Revolving Credit Facility due 2023
Credit facilities, short-term loan facility and long-term debt
Leverage ratio, actual (as a percent)		1.15%
Face amount		$ 125,000,000
Proceeds from credit facility		32,000,000
Debt, gross		$ 71,000,000	$ 39,000,000
PNGTS \| Secured debt \| Revolving Credit Facility due 2023 \| Maximum
Credit facilities, short-term loan facility and long-term debt
Leverage ratio, covenant (as a percent)		5.00%
PNGTS \| Secured debt \| Revolving Credit Facility due 2023 \| LIBOR
Credit facilities, short-term loan facility and long-term debt
Debt interest rate, at period end		1.42%	2.99%
Tuscarora \| Unsecured debt \| Unsecured Term Loan due 2021
Credit facilities, short-term loan facility and long-term debt
Debt, gross		$ 23,000,000	$ 23,000,000
Debt service coverage, actual		9.14%
Tuscarora \| Unsecured debt \| Unsecured Term Loan due 2021 \| Minimum
Credit facilities, short-term loan facility and long-term debt
Debt service coverage, covenant		3.00%
Tuscarora \| Unsecured debt \| Unsecured Term Loan due 2021 \| LIBOR
Credit facilities, short-term loan facility and long-term debt
Debt interest rate, at period end		1.30%	2.82%
North Baja \| Unsecured debt \| Unsecured Term Loan due 2021
Credit facilities, short-term loan facility and long-term debt
Face amount		$ 50,000,000
Percentage of debt to total capitalization, actual		40.50%
Debt, gross		$ 50,000,000	$ 50,000,000
North Baja \| Unsecured debt \| Unsecured Term Loan due 2021 \| Maximum
Credit facilities, short-term loan facility and long-term debt
Percentage of debt to total capitalization, covenant		70.00%
North Baja \| Unsecured debt \| Unsecured Term Loan due 2021 \| LIBOR
Credit facilities, short-term loan facility and long-term debt
Debt interest rate, at period end		1.25%	2.77%

DEBT AND CREDIT FACILITIES - Schedule of principal repayments required on debt (Details) $ in Millions	Jun. 30, 2020 USD ($)
Principal repayments required on debt
2020	$ 0
2021	423
2022	450
2023	71
2024	0
Thereafter	1,175
Total debt	$ 2,119

PARTNERS' EQUITY (Details) - USD ($)	3 Months Ended		6 Months Ended		12 Months Ended
PARTNERS' EQUITY (Details) - USD ($)	Mar. 31, 2020	Mar. 31, 2018	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
PARTNERS' EQUITY
Cash distribution			$ 8,000,000	$ 13,000,000
Distributions \| Class B Units
PARTNERS' EQUITY
Cash distribution	$ 8,000,000
TC Energy \| Distributions \| Class B Units
PARTNERS' EQUITY
Percentage of reduction in distributions payable			35.00%
TC Energy \| Distributions \| Common Units
PARTNERS' EQUITY
Percentage of reduction in distributions payable		35.00%
Minimum distribution payable per common unit (in dollars per unit)			$ 3.94
GTN \| TC Energy \| Distributions \| Class B Units
PARTNERS' EQUITY
Portion of annual distributions entitled to receive a percentage share of the distributions above threshold			30.00%
Percentage applied to 30 percent of GTN's distributions above threshold for the quarter ending March 31, 2020			100.00%
Threshold of GTN's total distributable cash flows for payment to Class B units for the quarter ending March 31, 2020			$ 20,000,000
Percentage applied to 30 percent of GTN's distributions above threshold thereafter			25.00%
Threshold of distributions for payment			$ 20,000,000		$ 20,000,000
Percentage of distributions above threshold			43.75%
Threshold of GTN's total distributable cash flows for payment to Class B units for the year ending December 31, 2020			$ 20,000,000
Percentage of reduction in distributions payable			35.00%

NET INCOME PER COMMON UNIT - Narrative (Details) - USD ($)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
General Partner \| General Partner \| TC Pipelines, LP
PARTNERS' EQUITY
General partner interest	2.00%	2.00%	2.00%	2.00%
Class B Units \| TC Energy \| Distributions
PARTNERS' EQUITY
Percentage of reduction in distributions payable			35.00%
Class B Units \| TC Energy \| GTN \| Distributions
PARTNERS' EQUITY
Portion of annual distributions entitled to receive a percentage share of the distributions above threshold			30.00%
Threshold of distributions for payment			$ 20,000,000		$ 20,000,000
Percentage of distributions above threshold			43.75%
Percentage of reduction in distributions payable			35.00%

NET INCOME PER COMMON UNIT - Schedule of net income per common unit (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Net income per common unit
Net income attributable to controlling interests	$ 57	$ 55	$ 145	$ 148
Net income attributable to the General Partner	(1)	(1)	(3)	(3)
Net income attributable to common units	$ 56	$ 54	$ 142	$ 145
Weighted average common units outstanding — basic and diluted (millions)	71.3	71.3	71.3	71.3
Net income per common unit - basic and diluted (in dollars per unit)	$ 0.78	$ 0.75	$ 1.99	$ 2.03

- Definition

The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

+ References + Details

CASH DISTRIBUTIONS PAID TO COMMON UNITS (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Common Units
Partners' Equity
Distribution paid (in dollars per unit)	$ 0.65	$ 0.65	$ 1.30	$ 1.30
Total cash distribution	$ 47	$ 47	$ 95	$ 95
General Partner \| General Partner
Partners' Equity
Total distribution for General Partner interest	$ 1	$ 1	$ 2	$ 2
General Partner \| TC Pipelines, LP \| General Partner
Partners' Equity
General partner interest	2.00%	2.00%	2.00%	2.00%

CHANGE IN OPERATING WORKING CAPITAL (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019
Supplemental Cash Flow Elements [Abstract]
Change in accounts receivable and other	$ 6	$ 15
Change in inventories	0	(1)
Change in other current assets	3	3
Change in accounts payable and accrued liabilities	0	(12)
Change in accounts payable to affiliates	(2)	0
Change in accrued interest	(1)	(1)
Change in operating working capital	$ 6	$ 4

RELATED PARTY TRANSACTIONS - Narrative (Details) MMBTU / d in Thousands, $ in Millions	3 Months Ended			6 Months Ended		12 Months Ended
	Jun. 30, 2020 USD ($)	Mar. 31, 2020 USD ($) MMBTU / d	Jun. 30, 2019 USD ($)	Jun. 30, 2020 USD ($)	Jun. 30, 2019 USD ($)	Dec. 31, 2018 USD ($)	Nov. 01, 2020 USD ($)	Dec. 31, 2019 USD ($)
General Partner \| Reimbursement of costs of services provided
Related Party Transaction [Line Items]
Costs charged	$ 1.0		$ 1.0	$ 2.0	$ 2.0
Great Lakes \| TC Energy
Related Party Transaction [Line Items]
Amount included in receivables from related party	$ 13.0			$ 13.0				$ 19.0
Great Lakes \| ANR Pipeline Company \| Transportation contracts
Related Party Transaction [Line Items]
Contract value		$ 182.0				$ 1,300.0
Contract term		20 years				15 years
Contract value terminated		$ 1,100.0
Related Party, Contract Capacity \| MMBTU / d		168
Great Lakes \| Customer concentration risk \| Total net revenues \| TC Energy \| Transportation contracts
Related Party Transaction [Line Items]
Percent of total revenues	75.00%		73.00%	74.00%	73.00%
PNGTS \| TC Energy \| Portland XPress expansion project (PXP), Phase III
Related Party Transaction [Line Items]
Development costs, reimbursable	$ 3.8			$ 3.8
PNGTS \| Affiliates \| Portland XPress expansion project (PXP), Phase III
Related Party Transaction [Line Items]
Phase in period				3 years
Forecast \| PNGTS \| TC Energy \| Portland XPress expansion project (PXP), Phase III
Related Party Transaction [Line Items]
Development costs, reimbursable							$ 9.1

RELATED PARTY TRANSACTIONS - Summary of related party transactions (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
Related Party Transaction [Line Items]
Accounts payable to affiliates	$ 6		$ 6		$ 8
Great Lakes \| TC Energy
Related Party Transaction [Line Items]
Accounts payable to affiliates	4		4		5
Great Lakes \| TC Energy \| Capital and operating costs
Related Party Transaction [Line Items]
Costs charged	11	$ 12	22	$ 23
Impact on the Partnership's net income attributable to controlling interests	5	5	$ 9	10
Percentage of capital and operating costs charged			100.00%
Northern Border \| TC Energy
Related Party Transaction [Line Items]
Accounts payable to affiliates	4		$ 4		4
Northern Border \| TC Energy \| Capital and operating costs
Related Party Transaction [Line Items]
Costs charged	10	10	20	19
Impact on the Partnership's net income attributable to controlling interests	4	5	$ 8	9
Percentage of capital and operating costs charged			100.00%
GTN \| TC Energy
Related Party Transaction [Line Items]
Accounts payable to affiliates	4		$ 4		5
GTN \| TC Energy \| Capital and operating costs
Related Party Transaction [Line Items]
Costs charged	12	11	24	21
Impact on the Partnership's net income attributable to controlling interests	7	8	15	16
Bison \| TC Energy
Related Party Transaction [Line Items]
Accounts payable to affiliates	0		0		0
Bison \| TC Energy \| Capital and operating costs
Related Party Transaction [Line Items]
Costs charged	1	0	1	1
Impact on the Partnership's net income attributable to controlling interests	1	0	1	1
North Baja \| TC Energy
Related Party Transaction [Line Items]
Accounts payable to affiliates	0		0		1
North Baja \| TC Energy \| Capital and operating costs
Related Party Transaction [Line Items]
Costs charged	1	2	2	3
Impact on the Partnership's net income attributable to controlling interests	1	1	2	2
Tuscarora \| TC Energy
Related Party Transaction [Line Items]
Accounts payable to affiliates	0		0		0
Tuscarora \| TC Energy \| Capital and operating costs
Related Party Transaction [Line Items]
Costs charged	1	1	2	2
Impact on the Partnership's net income attributable to controlling interests	1	1	2	2
PNGTS \| TC Energy
Related Party Transaction [Line Items]
Accounts payable to affiliates	1		1		$ 1
PNGTS \| TC Energy \| Capital and operating costs
Related Party Transaction [Line Items]
Costs charged	1	1	3	3
Impact on the Partnership's net income attributable to controlling interests	$ 1	$ 1	$ 2	$ 2
Percentage of capital and operating costs charged			100.00%

FAIR VALUE MEASUREMENTS (Details) - USD ($)	3 Months Ended		6 Months Ended
FAIR VALUE MEASUREMENTS (Details) - USD ($)	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
Interest rate derivatives
Net realized gain (loss) related to the interest rate swaps included in financial charges and other	$ (2,000,000)	$ 0	$ (2,000,000)	$ 1,000,000
Interest rate swaps \| Term loan \| 2013 Term Loan Facility due 2022
Interest rate derivatives
Weighted average fixed interest rate	3.26%		3.26%		3.26%
Hedges of cash flows \| Recurring fair value measurement \| Level 2 \| Interest rate swaps
Interest rate derivatives
Fair value of derivative liability, gross	$ 19,000,000		$ 19,000,000		$ 6,000,000
Fair value of derivative liability, net	19,000,000		19,000,000		6,000,000
Estimate of fair value measurement \| Level 2
Interest rate derivatives
Fair value of debt	$ 2,223,000,000		$ 2,223,000,000		$ 2,111,000,000

ACCOUNTS RECEIVABLE AND OTHER (Details) - USD ($) $ in Millions	Jun. 30, 2020	Dec. 31, 2019
Receivables [Abstract]
Trade accounts receivable, net of allowance of nil	$ 32	$ 37
Other	2	6
Accounts receivable and other	$ 34	$ 43

FINANCIAL CHARGES AND OTHER (Details) - USD ($)	3 Months Ended		6 Months Ended
FINANCIAL CHARGES AND OTHER (Details) - USD ($)	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Other Expense, Nonoperating [Abstract]
Interest expense	$ 20,000,000	$ 22,000,000	$ 40,000,000	$ 45,000,000
Net realized loss (gain) related to the interest rate swaps	2,000,000	0	2,000,000	(1,000,000)
Other income	(4,000,000)	(1,000,000)	(5,000,000)	(1,000,000)
Financial charges and other	$ 18,000,000	$ 21,000,000	$ 37,000,000	$ 43,000,000

SUBSEQUENT EVENTS (Details) - USD ($) $ / shares in Units, $ in Millions									3 Months Ended		6 Months Ended
	Sep. 29, 2020	Aug. 04, 2020	Aug. 01, 2020	Jul. 31, 2020	Jul. 23, 2020	Jul. 15, 2020	Jul. 09, 2020	Jul. 08, 2020	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Distributions
Cash distribution											$ 8	$ 13
Partnership's share of distributions											$ 115	$ 112
Northern Border
Distributions
Ownership interest									50.00%		50.00%
Great Lakes
Distributions
Ownership interest									46.45%	46.45%	46.45%	46.45%
General Partner \| TC Pipelines, LP \| General Partner
Distributions
General partner interest									2.00%	2.00%	2.00%	2.00%
Subsequent Events
Distributions
Cash distribution					$ 47
Subsequent Events \| Natural gas transmission IT application \| ANR Pipeline Company
Distributions
Intangible assets acquired			$ 51
Subsequent Events \| Two Percent interest
Distributions
Cash distribution, general partner					$ 1
Subsequent Events \| Northern Border
Distributions
Ownership interest				50.00%
Partnership's share of distributions				$ 5
Subsequent Events \| Great Lakes
Distributions
Ownership interest				46.45%
Partnership's share of distributions				$ 11
Subsequent Events \| General Partner \| TC Pipelines, LP \| General Partner
Distributions
General partner interest					2.00%
Subsequent Events \| Northern Border
Distributions
Limited partners, distribution declared							$ 11
Subsequent Events \| Great Lakes
Distributions
Limited partners, distribution declared						$ 24
Subsequent Events \| Iroquois
Distributions
Limited partners, distribution declared		$ 21
Subsequent Events \| PNGTS
Distributions
Limited partners, distribution declared								$ 14
Share of distributions to its non-controlling interest owner				$ 5
Subsequent Events \| TC Pipelines, LP \| Natural gas transmission IT application \| ANR Pipeline Company
Distributions
Intangible assets acquired			$ 38
Common Units \| Subsequent Events
Distributions
Distribution declared (in dollars per unit)					$ 0.65
Cash distribution					$ 46
Common Units \| Subsequent Events \| General Partner
Distributions
Cash distribution					$ 4
Number of units held by investor					5,797,106
Common Units \| Subsequent Events \| TC Energy
Distributions
Cash distribution					$ 7
Number of units held by investor					11,287,725
Forecast \| Iroquois
Distributions
Ownership interest	49.34%
Partnership's share of distributions	$ 10