NET INCOME 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		9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Net income (loss) per common unit
Net income attributable to controlling interests	$ 62	$ 54	$ 231	$ 186
Net income attributable to PNGTS' former parent				(2)
Net income attributable to General and Limited Partners	62	54	231	184
Incentive distributions attributable to the General Partner		(3)		(9)
Net income attributable to the General Partner and common units	58	43	227	167
Net income attributable to General Partner's two percent interest	(1)	(1)	(5)	(3)
Class B Units
Net income (loss) per common unit
Net income attributable to common units	4	8	4	8
Common Units
Net income (loss) per common unit
Net income attributable to common units	$ 57	$ 42	$ 222	$ 164
Weighted average common units outstanding - basic (in units)	71.3	69.4	71.3	68.9
Weighted average common units outstanding - diluted (in units)	71.3	69.4	71.3	68.9
Net income (loss) per common unit - basic (in dollars per unit)	$ 0.79	$ 0.61	$ 3.11	$ 2.38
Net income (loss) per common unit - diluted (in dollars per unit)	$ 0.79	$ 0.61	$ 3.11	$ 2.38
GTN | Class B Units | TransCanada
Distributions
Net income attributable to controlling interests	$ 4		$ 4
GTN | Class B Units | TransCanada | Distributions
Distributions
Percentage applied to GTN's distributable cash flow			30.00%	30.00%
Net income attributable to controlling interests	4	$ 8	$ 4	$ 8
30% of GTN's distributable cash flow			31	$ 28
Threshold of GTN's distributions for payment to Class B units	$ 20		$ 20		$ 20