EQUITY INVESTMENTS (Details) - USD ($) $ in Millions					1 Months Ended	3 Months Ended										7 Months Ended	12 Months Ended
		Dec. 31, 2017	Sep. 01, 2017	Jun. 01, 2017	Apr. 30, 2006	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016		Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Dec. 31, 2017	Dec. 31, 2017	Dec. 31, 2016		Dec. 31, 2015		Feb. 28, 2018	Feb. 01, 2018	Jan. 31, 2018	Oct. 01, 2017	Aug. 01, 2017	Jan. 31, 2017
EQUITY INVESTMENTS
Ownership interest (as a percent)																										49.34%
Equity Earnings						$ 37	$ 27	$ 24	$ 36	$ 22		$ 22	$ 20	$ 33			$ 124	$ 97	[1]	$ 97	[1]
Equity Investments		$ 1,213				1,213				918	[1]					$ 1,213	1,213	918	[1]
Impairment of equity-method investment	[1]																			199
Capital contribution to reduce the outstanding balance of revolver debt	[1]																			2
Return on investment distribution classified as investing activities																	5	0		0
LIABILITIES AND PARTNERS' EQUITY
Current portion of long-term debt (Note 8)		$ 51				51				52	[1]					51	51	52	[1]
Distributions from Equity Investments
Distributions received from equity investments																	145
Distributions from equity investments																	140	153	[1]	119	[1]
Return on investment distribution classified as investing activities																	$ 5	0		0
Great Lakes Settlement | FERC
EQUITY INVESTMENTS
Settlement rate reduced (as a percent)																									27.00%
Northern Border Settlement | FERC
EQUITY INVESTMENTS
Decrease in recourse rate on January 1, 2018 (as percent)		5.00%
Decrease in recourse rate on April 1, 2018 (as percent)		5.50%
Decrease in recourse rate beginning January 1, 2020 through December 31, 2023 (as percent)		2.00%
Decrease in overall recourse rate by January 1, 2020 (as percent)																	12.50%
Nonrecurring fair value measurement
EQUITY INVESTMENTS
Impairment of equity-method investment																	$ 0	0
Northern Border
EQUITY INVESTMENTS
Ownership interest (as a percent)			50.00%
Partnership interest held (as a percent)																	50.00%
Equity contribution			$ 83
Capital contribution to reduce the outstanding balance of revolver debt			$ 166
Great Lakes
EQUITY INVESTMENTS
Partnership interest held (as a percent)																	46.45%
Total cash call issued to fund debt repayment						10			9
LIABILITIES AND PARTNERS' EQUITY
Current portion of long-term debt (Note 8)		$ 19				19				19						19	$ 19	19
Iroquois
LIABILITIES AND PARTNERS' EQUITY
Current portion of long-term debt (Note 8)		$ 4				$ 4										4	$ 4
Revenues (expenses)
Transmission revenues																110
Operating expenses																(32)
Depreciation																(17)
Financial charges and other																(9)
Net income																$ 52
Northern Border
EQUITY INVESTMENTS
Ownership interest (as a percent)		50.00%				50.00%										50.00%	50.00%					50.00%		50.00%
Equity Earnings																	$ 67	69		66
Equity Investments		$ 512				$ 512				444						$ 512	512	444
Amortization period of transaction fee					12 years
Transaction fee					$ 10
Additional ownership interest acquired (as a percent)					20.00%
Equity contribution																	83
Undistributed earnings																	0	0		0
Amount of difference between the carrying value and the underlying equity in net assets resulting from the recognition and inclusion of goodwill		115				115				116						115	115	116
Assets
Cash and cash equivalents		14				14				14						14	14	14
Other current assets		36				36				36						36	36	36
Plant, property and equipment, net		1,063				1,063				1,089						1,063	1,063	1,089
Other assets		14				14				14						14	14	14
Assets, total		1,127				1,127				1,153						1,127	1,127	1,153
LIABILITIES AND PARTNERS' EQUITY
Current liabilities		38				38				38						38	38	38
Deferred credits and other		31				31				28						31	31	28
Net long-term debt, including current maturities		264				264				430						264	264	430
Current portion of long-term debt (Note 8)		0				0				0						0	0	0
Partners' equity
Partners' capital		795				795				659						795	795	659
Accumulated other comprehensive loss		(1)				(1)				(2)						(1)	(1)	(2)
Liabilities and Partners' Equity, total		$ 1,127				$ 1,127				1,153						$ 1,127	1,127	1,153
Revenues (expenses)
Transmission revenues																	291	292		286
Operating expenses																	(78)	(72)		(70)
Depreciation																	(59)	(59)		(60)
Financial charges and other																	(18)	(21)		(22)
Net income																	136	140		134
Northern Border | Nonrecurring fair value measurement
EQUITY INVESTMENTS
Impairment of equity-method investment																	$ 0
Northern Border | ONEOK Partners, L.P.
EQUITY INVESTMENTS
Ownership interest (as a percent)		50.00%				50.00%										50.00%	50.00%
Ownership interest (as a percent)		50.00%				50.00%										50.00%	50.00%
Great Lakes
EQUITY INVESTMENTS
Ownership interest (as a percent)		46.45%				46.45%										46.45%	46.45%						46.45%				46.45%
Equity Earnings																	$ 31	28		31
Equity Investments		$ 479				$ 479				474						$ 479	479	474
Equity contribution						5			$ 4								9	9	[1]	9	[1]
Undistributed earnings																	0	0		0
Assets
Current assets		107				107				66						107	107	66
Plant, property and equipment, net		701				701				714						701	701	714
Assets, total		808				808				780						808	808	780
LIABILITIES AND PARTNERS' EQUITY
Current liabilities		75				75				40						75	75	40
Net long-term debt, including current maturities		259				259				278						259	259	278
Other non-current liabilities		1				1										1	1
Partners' equity
Partners' capital		473				473				462						473	473	462
Liabilities and Partners' Equity, total		$ 808				$ 808				$ 780						$ 808	808	780
Revenues (expenses)
Transmission revenues																	181	179		177
Operating expenses																	(66)	(69)		(59)
Depreciation																	(29)	(28)		(28)
Financial charges and other																	(20)	(21)		(23)
Net income																	$ 66	$ 61		67
Great Lakes | Nonrecurring fair value measurement
EQUITY INVESTMENTS
Impairment of equity-method investment															$ 199					199
Amount of difference between the carrying value and the underlying equity in net assets resulting from the recognition and inclusion of goodwill															$ 260					260
Great Lakes | TransCanada
EQUITY INVESTMENTS
Ownership interest (as a percent)		53.55%				53.55%										53.55%	53.55%
Iroquois
EQUITY INVESTMENTS
Ownership interest (as a percent)		49.34%		49.34%		49.34%										49.34%	49.34%
Equity Earnings																	$ 26
Equity Investments		$ 222				$ 222										$ 222	222
Equity contribution				$ 710
Undistributed earnings																0
Amount of difference between the carrying value and the underlying equity in net assets resulting from the recognition and inclusion of goodwill		41				41										41	41
Return on investment distribution classified as investing activities																	5
Additional consideration on surplus cash				$ 28
Cash distribution paid																	27
Assets
Cash and cash equivalents		86				86										86	86
Other current assets		36				36										36	36
Plant, property and equipment, net		591				591										591	591
Other assets		8				8										8	8
Assets, total		721				721										721	721
LIABILITIES AND PARTNERS' EQUITY
Current liabilities		17				17										17	17
Net long-term debt, including current maturities		329				329										329	329
Other non-current liabilities		9				9										9	9
Partners' equity
Partners' capital		366				366										366	366
Liabilities and Partners' Equity, total		$ 721				$ 721										$ 721	721
Distributions from Equity Investments
Return on investment distribution classified as investing activities																	$ 5
TC PipeLines Intermediate Limited Partnership
EQUITY INVESTMENTS
Partnership interest held (as a percent)																	98.9899%
TC GL Intermediate Limited Partnership
EQUITY INVESTMENTS
Partnership interest held (as a percent)																	98.9899%
ASU 2016-15
EQUITY INVESTMENTS
Return on investment distribution classified as investing activities																				(25)
Distributions from Equity Investments
Distributions from equity investments																				25
Return on investment distribution classified as investing activities																				$ (25)

[1] Recast to consolidate PNGTS (Refer to Notes 2 and 7).