SUBSEQUENT EVENTS - Distributions (Details) - USD ($)																															3 Months Ended								9 Months Ended	12 Months Ended
	Feb. 12, 2016	Feb. 01, 2016	Jan. 21, 2016	Jan. 11, 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	Nov. 14, 2014	Oct. 23, 2014	Aug. 14, 2014	Jul. 23, 2014	May. 15, 2014	Apr. 25, 2014	Feb. 14, 2014	Jan. 16, 2014	Nov. 14, 2013	Oct. 24, 2013	Aug. 14, 2013	Jul. 23, 2013	May. 22, 2013	May. 15, 2013	Apr. 23, 2013	Feb. 14, 2013	Jan. 17, 2013	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013		Feb. 28, 2007
Distributions
Total cash distribution																															$ 59,000,000	$ 59,000,000	$ 55,000,000	$ 55,000,000	$ 55,000,000	$ 54,000,000	$ 52,000,000	$ 51,000,000
General Partner IDRs paid					$ 1,000,000		$ 1,000,000							$ 1,000,000																										$ 2,000,000	$ 1,000,000	$ 0
Northern Border
Distributions
Ownership interest (as a percent)																															50.00%								50.00%	50.00%
Great Lakes
Distributions
Ownership interest (as a percent)																															46.45%			46.45%					46.45%	46.45%				46.45%
Common units
Distributions
Per Unit Distribution, declared (in dollars per unit)						$ 0.89		$ 0.89		$ 0.84			$ 0.84		$ 0.84		$ 0.84		$ 0.81		$ 0.81		$ 0.81		$ 0.81			$ 0.78		$ 0.78
Distributions declared						$ 57,000,000		$ 56,000,000		$ 54,000,000			$ 54,000,000		$ 53,000,000		$ 53,000,000		$ 51,000,000		$ 50,000,000		$ 50,000,000		$ 50,000,000			$ 42,000,000		$ 42,000,000
Limited Partners, Distributions paid					$ 57,000,000		$ 56,000,000		$ 54,000,000			$ 54,000,000		$ 53,000,000		$ 53,000,000		$ 51,000,000		$ 50,000,000		$ 50,000,000		$ 50,000,000			$ 42,000,000		$ 42,000,000
Number of units																															64,300,000				63,600,000				64,300,000	64,300,000	63,600,000	62,300,000	[1]
Class B units | GTN | Distributions | TransCanada
Distributions
Percentage applied to GTN's distributable cash flow for the nine months ending December 31, 2015																																							30.00%
Prorated threshold of GTN's distributions for payment to Class B units																																							$ 15,000,000
Limited Partners | Common units
Distributions
Number of units																															64,317,449								64,317,449	64,317,449
Limited Partners | Common units | TC PipeLines GP, Inc.
Distributions
Number of units																															5,797,106								5,797,106	5,797,106
Limited Partners | Common units | TransCanada
Distributions
Number of units																															11,287,725								11,287,725	11,287,725
Limited Partners | Class B units | TransCanada
Distributions
Number of units																															1,900,000								1,900,000	1,900,000
General Partner | TC PipeLines GP, Inc.
Distributions
Ownership interest in the Partnership (as a percent)											2.00%															2.00%														2.00%	2.00%	2.00%
Subsequent event
Distributions
Total cash distribution	$ 59,000,000
General Partner IDRs paid	1,000,000
Subsequent event | Distribution declared | Northern Border
Distributions
Partnership distribution				$ 44,000,000
Subsequent event | Distribution declared | Great Lakes
Distributions
Partnership distribution				$ 42,000,000
Subsequent event | Cash Distribution Paid | Northern Border
Distributions
Partnership's share of distributions		$ 22,000,000
Subsequent event | Cash Distribution Paid | Great Lakes
Distributions
Partnership's share of distributions		$ 20,000,000
Subsequent event | TC PipeLines GP, Inc.
Distributions
General partner cash distributions	2,000,000
General Partner IDRs paid	1,000,000
Subsequent event | Common units
Distributions
Per Unit Distribution, declared (in dollars per unit)			$ 0.89
Distributions declared			$ 57,000,000
Limited Partners, Distributions paid	57,000,000
Subsequent event | Common units | TC PipeLines GP, Inc.
Distributions
Limited Partners, Distributions paid	5,000,000
Subsequent event | Common units | TransCanada
Distributions
Limited Partners, Distributions paid	10,000,000
Subsequent event | Class B units
Distributions
Distributions declared			$ 12,000,000
Limited Partners, Distributions paid	$ 12,000,000

[1] Recast as discussed in Note 2 and Note 6.