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			12 Months Ended
		Mar. 31, 2018	Mar. 31, 2017		Dec. 31, 2017
Net income (loss) per common unit
Net income attributable to controlling interests		$ 96	$ 77	[1]
Net income attributable to PNGTS' former parent	[1]		(2)
Net income allocable to General Partner and Limited Partners		96	75
Net income attributable to the General Partner		(2)	(1)
Incentive distributions attributable to the General Partner			(2)
Common Units
Net income (loss) per common unit
Net income attributable to common units		$ 94	$ 72	[1]
Weighted average common units outstanding - basic (in units)		71.2	68.3	[1]
Weighted average common units outstanding - diluted (in units)		71.2	68.3
Net income (loss) per common unit - basic (in dollars per unit)	[2]	$ 1.32	$ 1.05	[1]
Net income (loss) per common unit - diluted (in dollars per unit)		$ 1.32	$ 1.05
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
Threshold of GTN's distributions for payment to Class B units		$ 20			$ 20
Portion of GTN's annual distributions to which the Class B units are entitled to receive a percentage share of the distributions above threshold (as a percent)		30.00%

[1] Recast to consolidate PNGTS (Refer to Note 2).

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