QUARTERLY FINANCIAL DATA (unaudited) (Details) - USD ($) $ / shares in Units, $ in Millions														3 Months Ended								12 Months Ended
		Nov. 14, 2018	Aug. 15, 2018	May 15, 2018	Feb. 13, 2018	Nov. 14, 2017	Aug. 11, 2017	May 15, 2017	Feb. 14, 2017	Nov. 14, 2016	Aug. 12, 2016	May 13, 2016	Feb. 12, 2016	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, 2018	Dec. 31, 2017	Dec. 31, 2016		Oct. 16, 2018
Quarterly financial data (unaudited)
Transmission revenues														$ 220	$ 103	$ 111	$ 115	$ 109	$ 100	$ 101	$ 112	$ 549	$ 422	$ 426	[1]
Equity earnings														44	34	36	59	37	27	24	36	173	124	97	[1]
Net income (loss)														(406)	65	75	102	70	55	55	83
Net income (loss) attributable to controlling interests														$ (413)	$ 62	$ 73	$ 96	$ 66	$ 54	$ 55	$ 77
Net income (loss) per common unit (in dollars per unit)														$ (5.80)	$ 0.79	$ 1.00	$ 1.32	$ 0.77	$ 0.61	$ 0.73	$ 1.05
Cash distribution paid		$ 47	$ 47	$ 47	$ 91	$ 74	$ 74	$ 68	$ 90	$ 66	$ 65	$ 60	$ 71									218	$ 284	$ 250	[1]
Provision for revenue sharing																						$ 10
TC PipeLines GP, Inc. | General Partner
Quarterly financial data (unaudited)
General partner interest (as a percent)																						2.00%	2.00%	2.00%
Common Units
Quarterly financial data (unaudited)
Net income (loss) per common unit (in dollars per unit)	[2]																					$ (2.68)	$ 3.16	$ 3.21	[1]
Cash distribution paid														$ 47	$ 47	$ 47	$ 76	$ 74	$ 74	$ 68	$ 68
Class B Units
Quarterly financial data (unaudited)
Cash distribution paid																	$ 15				$ 22
GTN | FERC
Quarterly financial data (unaudited)
Provision for revenue sharing														$ 1	$ 9
Amount agreed to issue as refund to customers from January 1 to October 31, 2018																										$ 10
Bison
Quarterly financial data (unaudited)
Contract Termination Proceeds																						$ 97

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

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