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		6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Net income (loss) per common unit
Net income attributable to controlling interests	$ 73	$ 55	$ 169	$ 132
Net income attributable to PNGTS' former parent				(2)
Net income attributable to General and Limited Partners	73	55	169	130
Incentive distributions attributable to the General Partner		(3)		(5)
Net income attributable to General Partner's two percent interest	(1)	(2)	(3)	(3)
Net income attributable to common units	72	50	166	122
Common Units
Net income (loss) per common unit
Net income attributable to the General Partner and common units	73	52	169	125
Net income attributable to common units	$ 72	$ 50	$ 166	$ 122
Weighted average common units outstanding - basic (in units)	71.3	68.9	71.2	68.6
Weighted average common units outstanding - diluted (in units)	71.3	68.9	71.2	68.6
Net income (loss) per common unit - basic (in dollars per unit)	$ 1.00	$ 0.73	$ 2.33	$ 1.78
Net income (loss) per common unit - diluted (in dollars per unit)	$ 1.00	$ 0.73	$ 2.33	$ 1.78
GTN | Class B Units | TransCanada | Distributions
Distributions
Percentage applied to GTN's distributable cash flow			30.00%
Net income attributable to controlling interests	$ 0	$ 0	$ 0	$ 0
Threshold of GTN's distributions for payment to Class B units			$ 20		$ 20