CASH DISTRIBUTIONS - Distributions by Payment Date (Details) - USD ($)																													3 Months Ended								9 Months Ended	12 Months Ended
	Feb. 23, 2018	Feb. 13, 2018	Jan. 23, 2018	Nov. 14, 2017	Oct. 24, 2017	Aug. 11, 2017	Jul. 20, 2017	May 15, 2017	Apr. 25, 2017	Feb. 14, 2017	Jan. 23, 2017	Nov. 14, 2016	Oct. 20, 2016	Aug. 12, 2016	Jul. 21, 2016	May 13, 2016	Apr. 21, 2016	Feb. 12, 2016	Jan. 21, 2016	Nov. 13, 2015	Oct. 22, 2015	Aug. 14, 2015	Jul. 23, 2015	May 15, 2015	Apr. 23, 2015	Apr. 01, 2015	Feb. 13, 2015	Jan. 22, 2015	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Dec. 31, 2017	Dec. 31, 2016		Dec. 31, 2015
Partners' Equity
General Partner 2% paid				$ 1,000,000		$ 2,000,000		$ 1,000,000		$ 2,000,000		$ 1,000,000		$ 1,000,000		$ 1,000,000		$ 1,000,000		$ 1,000,000		$ 2,000,000		$ 1,000,000			$ 1,000,000
General Partner IDRs paid				3,000,000		3,000,000		2,000,000		2,000,000		2,000,000		2,000,000		1,000,000		1,000,000		1,000,000		1,000,000																$ 10,000,000	$ 6,000,000		$ 2,000,000
Total cash distributions				$ 74,000,000		$ 74,000,000		$ 68,000,000		$ 90,000,000		$ 66,000,000		$ 65,000,000		$ 60,000,000		$ 71,000,000		$ 59,000,000		$ 59,000,000		$ 55,000,000			$ 55,000,000											284,000,000	250,000,000	[1]	228,000,000	[1]
Common Units
Partners' Equity
Per Unit Distribution, declared (in dollars per unit)					$ 1.00		$ 1.00		$ 0.94		$ 0.94		$ 0.94		$ 0.94		$ 0.89		$ 0.89		$ 0.89		$ 0.89		$ 0.84			$ 0.84
Per Unit Distribution, paid (in dollars per unit)				$ 1.00		$ 1.00		$ 0.94		$ 0.94		$ 0.94		$ 0.94		$ 0.89		$ 0.89		$ 0.89		$ 0.89		$ 0.84			$ 0.84
Limited Partners, Distribution declared					$ 70,000,000		$ 69,000,000		$ 65,000,000		$ 64,000,000		$ 63,000,000		$ 62,000,000		$ 58,000,000		$ 57,000,000		$ 57,000,000		$ 56,000,000		$ 54,000,000			$ 54,000,000
Limited Partners, Distributions paid				$ 70,000,000		$ 69,000,000		$ 65,000,000		$ 64,000,000		$ 63,000,000		$ 62,000,000		$ 58,000,000		$ 57,000,000		$ 57,000,000		$ 56,000,000		$ 54,000,000			$ 54,000,000
Total cash distributions																													$ 74,000,000	$ 74,000,000	$ 68,000,000	$ 68,000,000	$ 66,000,000	$ 65,000,000	$ 60,000,000	$ 59,000,000
Class B Units
Partners' Equity
Limited Partners, Distribution declared											$ 22,000,000								$ 12,000,000
Limited Partners, Distributions paid										22,000,000								$ 12,000,000																				$ 22,000,000	12,000,000	[1]
Total cash distributions																																$ 22,000,000				$ 12,000,000
GTN | Class B Units | TransCanada | Distributions
Partners' Equity
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%
Percentage applied to GTN's distributable cash flow																		30.00%																			30.00%	30.00%
Threshold of GTN's distributions for payment to Class B units										$ 20,000,000								$ 15,000,000																			$ 15,000,000	$ 20,000,000	$ 20,000,000		$ 15,000,000
Subsequent Events
Partners' Equity
General Partner 2% paid		$ 2,000,000
General Partner IDRs paid		3,000,000
Total cash distributions		$ 91,000,000
Total Cash Distribution	$ 76,000,000
Subsequent Events | Common Units
Partners' Equity
Per Unit Distribution, declared (in dollars per unit)			$ 1.00
Per Unit Distribution, paid (in dollars per unit)		$ 1.00
Limited Partners, Distribution declared			$ 71,000,000
Limited Partners, Distributions paid		$ 71,000,000
Subsequent Events | Class B Units
Partners' Equity
Limited Partners, Distribution declared			15,000,000
Limited Partners, Distributions paid		15,000,000
Subsequent Events | GTN | Class B Units | TransCanada | Distributions
Partners' Equity
Threshold of GTN's distributions for payment to Class B units		$ 20,000,000	$ 20,000,000

[1] Recast to consolidate PNGTS (Refer to Notes 2 and 7).