QUARTERLY FINANCIAL DATA (unaudited) (Details) - USD ($) $ / shares in Units, $ in Millions															3 Months Ended								12 Months Ended
		Nov. 14, 2017	Aug. 11, 2017	May 15, 2017	Feb. 14, 2017	Nov. 14, 2016	Aug. 12, 2016	May 13, 2016	Feb. 12, 2016	Nov. 13, 2015	Aug. 14, 2015	May 15, 2015	Apr. 01, 2015	Feb. 13, 2015	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, 2017	Dec. 31, 2016		Dec. 31, 2015
Quarterly financial data (unaudited)
Transmission revenues															$ 109	$ 100	$ 101	$ 112	$ 111	$ 103	$ 101	$ 111	$ 422	$ 426	[1],[2]	$ 417	[1]
Equity earnings															37	27	24	36	22	22	20	33	124	97	[1]	97	[1]
Net income															70	55	55	83	65	60	57	81
Net income attributable to controlling interests															$ 66	$ 54	$ 55	$ 77	$ 61	$ 58	$ 55	$ 74	252	248	[1],[2]	37	[1]
Net income per common unit (in dollars per unit)															$ 0.77	$ 0.61	$ 0.73	$ 1.05	$ 0.70	$ 0.65	$ 0.76	$ 1.10
Cash distribution paid		$ 74	$ 74	$ 68	$ 90	$ 66	$ 65	$ 60	$ 71	$ 59	$ 59	$ 55		$ 55									$ 284	$ 250	[1]	$ 228	[1]
TC PipeLines GP, Inc. | General Partner
Quarterly financial data (unaudited)
General partner interest (as a percent)													2.00%										2.00%	2.00%		2.00%
Common Units
Quarterly financial data (unaudited)
Net income per common unit (in dollars per unit)	[3]																						$ 3.16	$ 3.21	[1]	$ (0.03)	[1]
Cash distribution paid															$ 74	$ 74	$ 68	$ 68	$ 66	$ 65	$ 60	$ 59
Class B Units
Quarterly financial data (unaudited)
Cash distribution paid																		$ 22				$ 12

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

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

[3] Net income per common unit prior to recast (Refer to Note 2).