NET INCOME (LOSS) PER COMMON UNIT- Terms of Class B Unit Distributions and Determination of Net Income (Loss) per Common Unit (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions		3 Months Ended								12 Months Ended
		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
Net income (loss) per common unit
Net income (loss) attributable to controlling interests										$ (182)	$ 252	$ 248	[1]
Net income attributable to PNGTS' former parent											(2)	(4)
Net income (loss) attributable to General and Limited Partners										(182)	250	244
Incentive distributions attributable to the General Partner											(12)	(7)
Net income attributable to the General Partner and common units										(195)	223	215
Net income (loss) attributable to General Partner's two percent interest										4	(4)	(4)
Net income (loss) per common unit - basic (in dollars per unit)		$ (5.80)	$ 0.79	$ 1.00	$ 1.32	$ 0.77	$ 0.61	$ 0.73	$ 1.05
Class B Units
Net income (loss) per common unit
Net income (loss) attributable to common units										13	15	22
Common Units
Net income (loss) per common unit
Net income (loss) attributable to common units										$ (191)	$ 219	$ 211	[1]
Weighted average common units outstanding - basic (in units)										71.3	69.2	65.7	[1]
Net income (loss) per common unit - basic (in dollars per unit)	[2]									$ (2.68)	$ 3.16	$ 3.21	[1]
GTN | Class B Units | TransCanada | Distributions
Net income (loss) per common unit
Net income (loss) attributable to controlling interests										$ (13)	$ (15)	$ (22)
Distributions
Percentage applied to GTN's distributable cash flow										30.00%
30% of GTN's distributable cash flow										$ 40
Threshold of GTN's distributions for payment to Class B units										20	$ 20	$ 20
Reduction in distribution										$ 7

[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).