SUBSEQUENT EVENTS - Distributions (Details) - USD ($) $ / shares in Units, $ in Millions										3 Months Ended			6 Months Ended
		Aug. 11, 2017	Aug. 01, 2017	Jul. 31, 2017	Jul. 27, 2017	Jul. 20, 2017	Jul. 18, 2017	Jul. 07, 2017	Jun. 01, 2017	Jun. 30, 2017	Jun. 30, 2016		Jun. 30, 2017	Jun. 30, 2016
Distributions
Partnership distribution	[1]												$ 160.0
Partnership's share of distributions													$ 68.0	$ 95.0	[2]
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%	11.81%	49.90%		11.81%	49.90%
Iroquois
Distributions
Ownership interest (as a percent)			49.34%
Portland Natural Gas Transmission System
Distributions
Ownership interest (as a percent)									11.81%
Total cash consideration									$ 53.0
Subsequent Events
Distributions
Total cash distribution		$ 74.0
Subsequent Events | Distribution declared | Northern Border
Distributions
Partnership distribution								$ 14.0
Subsequent Events | Distribution declared | Great Lakes
Distributions
Partnership distribution							$ 15.0
Subsequent Events | Distribution declared | Iroquois
Distributions
Partnership distribution					$ 28.0
Subsequent Events | Cash Distribution Paid | Northern Border
Distributions
Partnership's share of distributions				$ 7.0
Subsequent Events | Cash Distribution Paid | Great Lakes
Distributions
Partnership's share of distributions			$ 7.0
Subsequent Events | Cash Distribution Paid | Iroquois
Distributions
Partnership's share of distributions			$ 14.0
General Partner
Distributions
Partnership distribution	[1]												$ 6.0
TC PipeLines GP, Inc. | Subsequent Events
Distributions
General Partner cash distributions						$ 5.0
Ownership interest (as a percent)						2.00%
Total distribution for General Partner interest						$ 2.0
TC PipeLines GP, Inc. | General Partner
Distributions
Total distribution for General Partner interest										$ 1.0	$ 1.0		2.0	$ 2.0
TC PipeLines GP, Inc. | General Partner | Subsequent Events | Distribution declared
Distributions
Distribution declared for IDRs						$ 3.0
Common Units
Distributions
Total cash distribution										$ 68.0	$ 60.0		$ 135.0	$ 119.0
Number of units										69,000,000	65,900,000	[2]	69,000,000	65,900,000	[2]
Common Units | Subsequent Events
Distributions
Per Unit Distribution, declared (in dollars per unit)		$ 0.06				$ 1.00
Limited partners, Distribution declared						$ 69.0
Common Units | Limited Partners
Distributions
Partnership distribution	[1]												$ 129.0
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 | TransCanada | Subsequent Events
Distributions
Limited Partners, Distributions paid		$ 11.0
Common Units | TransCanada | TC PipeLines GP, Inc.
Distributions
Number of units						11,287,725

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

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