SUBSEQUENT EVENTS - Distributions (Details) - USD ($) $ / shares in Units, $ in Millions						3 Months Ended
	May 15, 2018	May 01, 2018	Apr. 30, 2018	Apr. 16, 2018	Apr. 12, 2018	Mar. 31, 2018		Mar. 31, 2017		Jun. 01, 2017
Distributions
Partnership distribution						$ 93.0
Partnership's share of distributions						$ 43.0		$ 28.0	[1]
Northern Border
Distributions
Ownership interest (as a percent)			50.00%			50.00%	[2]
Great Lakes
Distributions
Ownership interest (as a percent)		46.45%				46.45%
Portland Natural Gas Transmission System
Distributions
Ownership interest (as a percent)										61.71%
Subsequent Events
Distributions
Total cash distribution		$ 47.0
Percentage of reduction in cash distributions		35.00%
Subsequent Events | Distribution declared | Northern Border
Distributions
Partnership distribution					$ 8.8
Subsequent Events | Distribution declared | Great Lakes
Distributions
Partnership distribution				$ 54.8
Subsequent Events | Cash Distribution Paid | Northern Border
Distributions
Partnership's share of distributions			$ 4.4
Subsequent Events | Cash Distribution Paid | Great Lakes
Distributions
Partnership's share of distributions		$ 25.5
General Partner
Distributions
Partnership distribution						$ 5.0
TC PipeLines GP, Inc. | Subsequent Events
Distributions
General Partner cash distributions	$ 1.0
Ownership interest (as a percent)		2.00%
Common Units
Distributions
Total cash distribution						$ 76.0		$ 68.0
Number of units						71,300,000		68,600,000	[1]
Common Units | Subsequent Events
Distributions
Per Unit Distribution, declared (in dollars per unit)		$ 0.65
Limited partners, Distribution declared	46.0
Reduction in distribution per common unit		$ 1.00
Common Units | Limited Partners
Distributions
Partnership distribution						$ 71.0
Common Units | TC PipeLines GP, Inc. | Subsequent Events
Distributions
Limited Partners, Distributions paid	4.0
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	$ 7.0
Common Units | TransCanada | TC PipeLines GP, Inc.
Distributions
Number of units		11,287,725

[1] Recast to consolidate PNGTS (Refer to Note 2).

[2] OwnershipEquity EarningsEquity InvestmentsInterest atThree months(unaudited)March 31, ended March 31, March 31, December 31, (millions of dollars)2018    2018201720182017Northern Border (a) 50% 17  19 507 512Great Lakes 46.45% 24 17 499 479Iroquois(b) 49.34% 18  — 211 222  59  36 1,217 1,213Equity earnings from Northern Border is net of the 12-year amortization of a $10 million transaction fee paid to the operator of Northern Border at the time of the Partnership’s acquisition of an additional 20 percent interest in April 2006.