DEBT AND CREDIT FACILITIES - Schedule of Debt (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	May 25, 2017
Debt and credit facilities
Total credit facilities, short-term loan facility and long-term debt	$ 2,198	$ 2,012
Less: unamortized debt issuance costs and debt discount	7	9
Current portion of long-term debt (Note 8)	423	123
Long-term debt	1,768	1,880
TC PipeLines, LP Senior Credit Facility due 2021
Debt and credit facilities
Debt and credit facilities	$ 0	$ 0
Weighted average interest rate (as a percent)	0.00%	0.00%
TC PipeLines, LP 2013 Term Loan Facility due 2022
Debt and credit facilities
Debt and credit facilities	$ 450	$ 450
Weighted average interest rate (as a percent)	1.87%	3.52%
TC PipeLines, LP 4.65% Senior Notes due 2021
Debt and credit facilities
Stated interest rate	4.65%	4.65%
Debt and credit facilities	$ 350	$ 350
Weighted average interest rate (as a percent)	4.65%	4.65%
TC PipeLines, LP 4.375% Senior Notes due 2025
Debt and credit facilities
Stated interest rate	4.375%	4.375%
Debt and credit facilities	$ 350	$ 350
Weighted average interest rate (as a percent)	4.375%	4.375%
TC PipeLines, LP 3.90% Senior Notes due 2027
Debt and credit facilities
Stated interest rate	3.90%	3.90%	3.90%
Debt and credit facilities	$ 500	$ 500
Weighted average interest rate (as a percent)	3.90%	3.90%
GTN 5.29% Senior Notes due 2020
Debt and credit facilities
Stated interest rate	5.29%	5.29%
Debt and credit facilities	$ 0	$ 100
Weighted average interest rate (as a percent)	0.00%	5.29%
GTN 5.69% Senior Notes due 2035
Debt and credit facilities
Stated interest rate	5.69%	5.69%
Debt and credit facilities	$ 150	$ 150
Weighted average interest rate (as a percent)	5.69%	5.69%
G T N Series A Senior Notes due 2030
Debt and credit facilities
Stated interest rate	3.12%
Debt and credit facilities	$ 175	$ 0
Weighted average interest rate (as a percent)	3.12%	0.00%
PNGTS Revolving Credit Facility due 2023
Debt and credit facilities
Debt and credit facilities	$ 25	$ 39
Weighted average interest rate (as a percent)	1.88%	3.47%
PNGTS Series A Senior Notes due 2030
Debt and credit facilities
Stated interest rate	2.84%
Debt and credit facilities	$ 125	$ 0
Weighted average interest rate (as a percent)	2.84%	0.00%
Tuscarora Term Loan due 2020
Debt and credit facilities
Debt and credit facilities	$ 23	$ 23
Weighted average interest rate (as a percent)	2.13%	3.39%
North Baja Term Loan Due 2021
Debt and credit facilities
Debt and credit facilities	$ 50	$ 50
Weighted average interest rate (as a percent)	1.70%	3.34%

Amount, before unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt.

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