DEBT AND CREDIT FACILITIES - Amounts Outstanding and Description of Terms (Details) - USD ($) $ in Millions					12 Months Ended
	Jun. 26, 2019	Apr. 05, 2018	Sep. 29, 2017	May 25, 2017	Dec. 31, 2019	Dec. 31, 2018	Dec. 19, 2018
Credit facilities, short-term loan facility and long-term debt
Total credit facilities, short-term loan facility and long-term debt					$ 2,012	$ 2,118
Less: unamortized debt issuance costs and debt discount					9	10
Less: current portion					123	36
Long-term debt					1,880	2,072
2013 Term Loan Facility
Credit facilities, short-term loan facility and long-term debt
Repayments of secured debt	$ 50
Unwind rate of interest rate swap (as a percent)	2.81%
TC PipeLines, LP Senior Credit Facility due 2021
Credit facilities, short-term loan facility and long-term debt
Debt and credit facilities						$ 40
Weighted average interest rate (as a percent)						3.14%
Maximum borrowing capacity					500
Amount outstanding under credit facility					0	$ 40
Remaining borrowing capacity					$ 500
Effective interest rate (as a percent)					3.77%
Increase in credit facility					$ 500
TC PipeLines, LP 2013 Term Loan Facility due 2022
Credit facilities, short-term loan facility and long-term debt
Debt and credit facilities					$ 450	$ 500
Weighted average interest rate (as a percent)					3.52%	3.23%
Effective interest rate (as a percent)					2.94%	3.60%
Leverage ratio, actual (as a percent)					3.41%
TC PipeLines, LP 2013 Term Loan Facility due 2022 | Fiscal quarter and the two following fiscal quarters in which one or more acquisitions has been executed
Credit facilities, short-term loan facility and long-term debt
Leverage ratio, covenant (as a percent)					5.50%
TC PipeLines, LP 2013 Term Loan Facility due 2022 | Debt agreement covenants, periods subsequent to initial period after occurrence of acquisition
Credit facilities, short-term loan facility and long-term debt
Additional period immediately following the fiscal quarter in which a specified material acquisition occurs					6 months
Leverage ratio, covenant (as a percent)					5.00%
TC PipeLines, LP 2013 Term Loan Facility due 2022 | Hedges of cash flows | Interest rate swaps
Credit facilities, short-term loan facility and long-term debt
Effective interest rate (as a percent)					3.26%	3.26%
TC PipeLines, LP 2013 Term Loan Facility due 2022 | Federal funds rate
Credit facilities, short-term loan facility and long-term debt
Basis spread on variable rate (as a percent)			0.50%
TC PipeLines, LP 2013 Term Loan Facility due 2022 | LIBOR
Credit facilities, short-term loan facility and long-term debt
Basis spread on variable rate (as a percent)			1.00%
TC PipeLines, LP 2013 Term Loan Facility due 2022 | LIBOR | Minimum
Credit facilities, short-term loan facility and long-term debt
Basis spread on variable rate (as a percent)			1.125%
TC PipeLines, LP 2013 Term Loan Facility due 2022 | LIBOR | Maximum
Credit facilities, short-term loan facility and long-term debt
Basis spread on variable rate (as a percent)			2.00%
TC PipeLines, LP 2013 Term Loan Facility due 2022 | Base rate | Minimum
Credit facilities, short-term loan facility and long-term debt
Basis spread on variable rate (as a percent)			0.125%
TC PipeLines, LP 2013 Term Loan Facility due 2022 | Base rate | Maximum
Credit facilities, short-term loan facility and long-term debt
Basis spread on variable rate (as a percent)			1.00%
TC PipeLines, LP 4.65% Senior Notes due 2021
Credit facilities, short-term loan facility and long-term debt
Debt and credit facilities					$ 350	$ 350
Stated interest rate (as a percent)					4.65%	4.65%
Weighted average interest rate (as a percent)					4.65%	4.65%
TC PipeLines, LP 4.375% Senior Notes due 2025
Credit facilities, short-term loan facility and long-term debt
Debt and credit facilities					$ 350	$ 350
Stated interest rate (as a percent)					4.375%	4.375%
Weighted average interest rate (as a percent)					4.375%	4.375%
TC PipeLines, LP 3.90% Senior Notes due 2027
Credit facilities, short-term loan facility and long-term debt
Debt and credit facilities					$ 500	$ 500
Stated interest rate (as a percent)				3.90%	3.90%	3.90%
Weighted average interest rate (as a percent)					3.90%	3.90%
Amount of debt				$ 500
Net proceeds				$ 497
GTN 5.29% Senior Notes due 2020
Credit facilities, short-term loan facility and long-term debt
Debt and credit facilities					$ 100	$ 100
Stated interest rate (as a percent)					5.29%	5.29%
Weighted average interest rate (as a percent)					5.29%	5.29%
GTN 5.69% Senior Notes due 2035
Credit facilities, short-term loan facility and long-term debt
Debt and credit facilities					$ 150	$ 150
Stated interest rate (as a percent)					5.69%	5.69%
Weighted average interest rate (as a percent)					5.69%	5.69%
GTN Unsecured Term Loan Facility due 2019
Credit facilities, short-term loan facility and long-term debt
Debt and credit facilities						$ 35
Weighted average interest rate (as a percent)						2.93%
Effective interest rate (as a percent)						3.30%
PNGTS Revolving Credit Facility due 2023
Credit facilities, short-term loan facility and long-term debt
Debt and credit facilities					$ 39	$ 19
Weighted average interest rate (as a percent)					3.47%	3.55%
Maximum borrowing capacity		$ 125
Amount outstanding under credit facility					$ 39
Leverage ratio, covenant (as a percent)		5.00%
Leverage ratio, actual (as a percent)					0.70%
Interest rate (as a percent)					2.99%	3.60%
Tuscarora Term Loan due 2020
Credit facilities, short-term loan facility and long-term debt
Debt and credit facilities					$ 23	$ 24
Weighted average interest rate (as a percent)					3.39%	3.10%
Effective interest rate (as a percent)					2.82%	3.47%
Debt service coverage, covenant (as a percent)					3.00%
Debt service coverage, actual (as a percent)					8.72%
North Baja Term Loan Due 2021
Credit facilities, short-term loan facility and long-term debt
Debt and credit facilities					$ 50	$ 50
Weighted average interest rate (as a percent)					3.34%	3.54%
Effective interest rate (as a percent)					2.77%	3.54%
Percentage of debt to total capitalization, covenant					70.00%
Percentage of debt to total capitalization, actual					39.80%
Amount of debt							$ 50
GTN Unsecured Senior Notes
Credit facilities, short-term loan facility and long-term debt
Percentage of debt to total capitalization, covenant					70.00%
Percentage of debt to total capitalization, actual					39.10%