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, 2017	Sep. 30, 2016		Jun. 30, 2016	Sep. 30, 2017	Sep. 30, 2016		Dec. 31, 2016
Net income (loss) per common unit
Net income attributable to controlling interests	$ 54	$ 58	[1]		$ 186	$ 188	[1]
Net income attributable to PNGTS' former parent					(2)	(3)
Net income attributable to General and Limited Partners	54	58			184	185
Incentive distributions allocated to the General Partner	(3)	(2)			(9)	(5)
Net income attributable to the General Partner and common units	43	45			167	168
Net income attributable to General Partner's effective two percent interest	(1)	(2)			(3)	(4)
Class B Units
Net income (loss) per common unit
Net income attributable to limited partners	(8)	(11)			(8)	(12)
Common Units
Net income (loss) per common unit
Net income attributable to limited partners	$ 42	$ 43	[1]		$ 164	$ 164	[1]
Weighted average common units outstanding - basic (in units)	69.4	66.1	[1]		68.9	65.3	[1]
Weighted average common units outstanding - diluted (in units)	69.4	66.1			68.9	65.3
Net income per common unit - basic (in dollars per unit)	$ 0.61	$ 0.65	[1],[2]		$ 2.38	$ 2.51	[1],[2]
Net income per common unit - diluted (in dollars per unit)	$ 0.61	$ 0.65			$ 2.38	$ 2.51
GTN | Class B Units
Distributions
30% of GTN's distributable cash flow					$ 28
GTN | Class B Units | TransCanada | Distributions
Net income (loss) per common unit
Net income attributable to controlling interests	$ 8	$ 11		$ 1	$ 8	$ 12
Distributions
Percentage applied to GTN's distributable cash flow for the twelve month period ending December 31, 2017					30.00%
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%				30.00%	30.00%
30% of GTN's distributable cash flow	$ 28				$ 28	$ 32

[1] Recast to consolidate PNGTS for all periods presented (Refer to Notes 2 and 6).

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