SUBSEQUENT EVENTS - Distributions (Details) - USD ($) $ / shares in Units, $ in Millions																																					12 Months Ended
		Feb. 28, 2018	Feb. 23, 2018	Feb. 14, 2018	Feb. 13, 2018	Feb. 01, 2018	Jan. 31, 2018	Jan. 23, 2018	Jan. 22, 2018	Jan. 10, 2018	Jan. 08, 2018	Jan. 02, 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	Feb. 13, 2015	Jan. 22, 2015	Dec. 31, 2017	Dec. 31, 2016		Dec. 31, 2015		Sep. 01, 2017	Aug. 01, 2017	Jun. 01, 2017	Jan. 31, 2017	Jan. 01, 2016
Distributions
Ownership interest (as a percent)																																											49.34%
General Partner IDRs paid													$ 3.0		$ 3.0		$ 2.0		$ 2.0		$ 2.0		$ 2.0		$ 1.0		$ 1.0		$ 1.0		$ 1.0						$ 10.0	$ 6.0		$ 2.0
Partnership distribution	[1]																																				310.0	276.0		268.0
Partnership's share of distributions																																					140.0	153.0	[2]	119.0	[2]
Return on investment distribution classified as investing activities																																					$ 5.0	0.0		0.0
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%
Portland Natural Gas Transmission System
Distributions
Ownership interest (as a percent)																																												11.81%		49.90%
Iroquois
Distributions
Ownership interest (as a percent)						49.34%
Northern Border
Distributions
Ownership interest (as a percent)																																										50.00%
Portland Natural Gas Transmission System
Distributions
Ownership interest (as a percent)																																					61.71%
Portland Natural Gas Transmission System | Note Purchase Agreement | 2003 Senior Secured Notes
Distributions
Total amount due on senior secured notes												$ 6.0
Payment of principal amount on secured notes												6.0
Payment of interest on secured notes												$ 0.0
Subsequent Events
Distributions
Total cash distribution			$ 76.0
General Partner IDRs paid					$ 3.0
Subsequent Events | Distribution declared | Northern Border
Distributions
Partnership distribution											$ 15.0
Subsequent Events | Distribution declared | Great Lakes
Distributions
Limited partners, Distribution declared										$ 20.0
Subsequent Events | Distribution declared | Iroquois
Distributions
Limited partners, Distribution declared									$ 29.0
Subsequent Events | Cash Distribution Paid | Northern Border
Distributions
Limited partners, Distribution declared				$ 17.0
Partnership's share of distributions		$ 9.0					$ 7.0
Subsequent Events | Cash Distribution Paid | Great Lakes
Distributions
Partnership's share of distributions						$ 9.0
Subsequent Events | Cash Distribution Paid | Iroquois
Distributions
Partnership's share of distributions						14.0
Return on investment distribution classified as investing activities						$ 2.6
General Partner
Distributions
Partnership distribution																																					$ 16.0	$ 10.0	[1]	$ 7.0	[1]
TC PipeLines GP, Inc. | Subsequent Events
Distributions
General Partner cash distributions								$ 5.0
Total distribution for General Partner interest								$ 2.0
Ownership interest (as a percent)								2.00%
TC PipeLines GP, Inc. | General Partner | Subsequent Events | Distribution declared
Distributions
General Partner IDRs paid								$ 3.0
Common Units
Distributions
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
Limited Partners, Distributions paid													$ 70.0		$ 69.0		$ 65.0		$ 64.0		$ 63.0		$ 62.0		$ 58.0		$ 57.0		$ 57.0		$ 56.0		$ 54.0		$ 54.0
Number of units																																					70,600,000	67,400,000	[2],[3]	64,300,000	[2]
Limited partners, Distribution declared														$ 70.0		$ 69.0		$ 65.0		$ 64.0		$ 63.0		$ 62.0		$ 58.0		$ 57.0		$ 57.0		$ 56.0		$ 54.0		$ 54.0
Common Units | Subsequent Events
Distributions
Per Unit Distribution, declared (in dollars per unit)								$ 1.00
Limited Partners, Distributions paid					71.0
Limited partners, Distribution declared								$ 71.0
Common Units | Limited Partners
Distributions
Number of units																																					70,573,423
Partnership distribution																																					$ 268.0	$ 240.0	[1]	$ 221.0	[1]
Common Units | TC PipeLines GP, Inc. | Subsequent Events
Distributions
Limited Partners, Distributions paid					6.0
Common Units | TC PipeLines GP, Inc. | Limited Partners
Distributions
Number of units																																					5,797,106
Common Units | TC PipeLines GP, Inc. | Limited Partners | Subsequent Events
Distributions
Number of units								5,797,106
Common Units | TransCanada | Subsequent Events
Distributions
Limited Partners, Distributions paid					$ 11.0
Common Units | TransCanada | Limited Partners
Distributions
Number of units																																					11,287,725
Common Units | TransCanada | Limited Partners | Subsequent Events
Distributions
Number of units								11,287,725

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

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

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