EQUITY INVESTMENTS (Details) - USD ($) $ in Millions			1 Months Ended		3 Months Ended								6 Months Ended	7 Months Ended	12 Months Ended
	Sep. 01, 2017	Jun. 01, 2017	Jun. 30, 2017	Apr. 30, 2006	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2017	Dec. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016		Aug. 01, 2017
EQUITY INVESTMENTS
Equity Earnings					$ 44	$ 34	$ 36	$ 59	$ 37	$ 27	$ 24	$ 36			$ 173	$ 124	$ 97	[1]
Equity Investments					1,196				1,213				$ 1,213	$ 1,213	1,196	1,213
LIABILITIES AND PARTNERS' EQUITY
Current portion of long-term debt (Note 9)					$ 36				51				51	51	36	51
Distributions from Equity Investments
Distributions received from equity investments															198	145	153
Return on investment distribution classified as investing activities															$ 10	5	0
Northern Border
EQUITY INVESTMENTS
Ownership interest (as a percent)					50.00%										50.00%
Partnership interest held (as a percent)															50.00%
Great Lakes
EQUITY INVESTMENTS
Partnership interest held (as a percent)															46.45%
Great Lakes | Minimum
EQUITY INVESTMENTS
Excess of estimated fair value over carrying value (as a percent)					10.00%										10.00%
Nonrecurring fair value measurement
EQUITY INVESTMENTS
Equity Method Investment, Other than Temporary Impairment															$ 0	0
ONEOK Partners, L.P. | Northern Border
EQUITY INVESTMENTS
Ownership interest (as a percent)					50.00%										50.00%
Ownership interest (as a percent)					50.00%										50.00%
Northern Border
EQUITY INVESTMENTS
Ownership interest (as a percent)	50.00%
Equity contribution	$ 83
Capital contribution to reduce the outstanding balance of revolver debt	$ 166
Great Lakes
EQUITY INVESTMENTS
Total cash call issued to fund debt repayment					$ 10			9
LIABILITIES AND PARTNERS' EQUITY
Current portion of long-term debt (Note 9)					21				19				19	19	$ 21	19
Iroquois
EQUITY INVESTMENTS
Cash distribution paid															56	27
Assets
Cash and cash equivalents					80										80
Other current assets					32										32
Property, plant and equipment, net					581										581
Other assets					8										8
Assets, total					701										701
LIABILITIES AND PARTNERS' EQUITY
Current liabilities					19										19
Net long-term debt, net					325										325
Current portion of long-term debt (Note 9)					146				$ 4				$ 4	4	146	$ 4
Other non-current liabilities					14										14
Partners' capital					343										343
Liabilities and Partners' Equity, total					$ 701										701
Revenues (expenses)
Transmission revenues														110	194
Operating expenses														(32)	(57)
Depreciation														(17)	(29)
Financial charges and other														(9)	(14)
Net income														$ 52	94
Distributions from Equity Investments
Return on investment distribution classified as investing activities															$ 10
Great Lakes
EQUITY INVESTMENTS
Ownership interest (as a percent)					46.45%										46.45%
Percentage less than of which fair value exceeded carrying value									10.00%				10.00%	10.00%		10.00%
Equity Earnings															$ 59	$ 31	28
Equity Investments					$ 489				$ 479				$ 479	$ 479	489	479
Equity contribution					5			$ 4							9	9	9	[1]
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					260				260				260	260	260	260
Assets
Current assets					75				107				107	107	75	107
Property, plant and equipment, net					689				701				701	701	689	701
Assets, total					764				808				808	808	764	808
LIABILITIES AND PARTNERS' EQUITY
Current liabilities					26				75				75	75	26	75
Net long-term debt, net					240				259				259	259	240	259
Other non-current liabilities					4				1				1	1	4	1
Partners' capital					494				473				473	473	494	473
Liabilities and Partners' Equity, total					$ 764				808				808	808	764	808
Revenues (expenses)
Transmission revenues															246	181	179
Operating expenses															(68)	(66)	(69)
Depreciation															(32)	(29)	(28)
Financial charges and other															(18)	(20)	(21)
Net income															$ 128	66	61
Great Lakes | TransCanada
EQUITY INVESTMENTS
Ownership interest (as a percent)					53.55%										53.55%
Iroquois
EQUITY INVESTMENTS
Ownership interest (as a percent)		49.34%			49.34%										49.34%				49.34%
Equity Earnings															$ 46	26
Equity Investments					$ 210				222				222	222	210	222
Equity contribution		$ 710
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					$ 41				41				41	41	41	41
Additional consideration on surplus cash		$ 28
Assets
Cash and cash equivalents									86				86	86		86
Other current assets									36				36	36		36
Property, plant 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, net									329				329	329		329
Other non-current liabilities									9				9	9		9
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															$ 10	5
TC PipeLines Intermediate Limited Partnership | Northern Border
EQUITY INVESTMENTS
Partnership interest held (as a percent)															100.00%
TC PipeLines Intermediate Limited Partnership | Great Lakes
EQUITY INVESTMENTS
Partnership interest held (as a percent)															100.00%
Northern Border
EQUITY INVESTMENTS
Ownership interest (as a percent)					50.00%										50.00%
Equity Earnings															$ 68	67	69
Equity Investments					$ 497				512				512	512	497	512
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				115	115	115	115
Assets
Cash and cash equivalents					10				14				14	14	10	14
Other current assets					36				36				36	36	36	36
Property, plant and equipment, net					1,037				1,063				1,063	1,063	1,037	1,063
Other assets					13				14				14	14	13	14
Assets, total					1,096				1,127				1,127	1,127	1,096	1,127
LIABILITIES AND PARTNERS' EQUITY
Current liabilities					34				38				38	38	34	38
Deferred credits and other					35				31				31	31	35	31
Net long-term debt, net					264				264				264	264	264	264
Current portion of long-term debt (Note 9)					0										0
Partners' capital					764				795				795	795	764	795
Accumulated other comprehensive loss					(1)				(1)				(1)	(1)	(1)	(1)
Liabilities and Partners' Equity, total					$ 1,096				$ 1,127				$ 1,127	$ 1,127	1,096	1,127
Revenues (expenses)
Transmission revenues															289	291	292
Operating expenses															(78)	(78)	(72)
Depreciation															(60)	(59)	(59)
Financial charges and other															(15)	(18)	(21)
Net income															$ 136	$ 136	$ 140

[1] Recast to consolidate PNGTS (Refer to Notes 2 and 8).