SUBSEQUENT EVENTS - Distributions (Details) - USD ($) $ / shares in Units, $ in Millions								3 Months Ended				6 Months Ended
	Aug. 15, 2018	Aug. 01, 2018	Jul. 31, 2018	Jul. 26, 2018	Jul. 20, 2018	Jul. 17, 2018	Jul. 11, 2018	Jun. 30, 2018		Dec. 31, 2017	Jun. 30, 2017	Jun. 30, 2018		Jun. 30, 2017	Mar. 31, 2018
Distributions
Partnership distribution												$ 141.0
Partnership's share of distributions												$ 96.0		$ 68.0
Northern Border
Distributions
Ownership interest (as a percent)			50.00%					50.00%	[1]			50.00%	[1]
Great Lakes
Distributions
Ownership interest (as a percent)		46.45%						46.45%				46.45%			46.45%
Subsequent Events
Distributions
Total cash distribution				$ 47.0
Percentage of reduction in cash distributions				35.00%
Subsequent Events | Distribution declared | Portland Natural Gas Transmission System
Distributions
Partnership distribution					$ 3.0
Subsequent Events | Distribution declared | Northern Border
Distributions
Partnership distribution							$ 12.0
Subsequent Events | Distribution declared | Great Lakes
Distributions
Partnership distribution						$ 29.0
Subsequent Events | Cash Distribution Paid | Northern Border
Distributions
Partnership's share of distributions			$ 6.0
Subsequent Events | Cash Distribution Paid | Great Lakes
Distributions
Partnership's share of distributions		$ 13.0
Subsequent Events | Cash Distribution Paid | Portland Natural Gas Transmission System
Distributions
Share of distributions to its non-controlling interest owner		$ 1.0
General Partner
Distributions
Partnership distribution												$ 6.0
TC PipeLines GP, Inc. | Subsequent Events
Distributions
General Partner cash distributions	$ 1.0
Ownership interest (as a percent)				2.00%
Common Units
Distributions
Per Unit Distribution, declared (in dollars per unit)										$ 1.00
Total cash distribution								$ 47.0			$ 68.0	$ 123.0		$ 135.0
Number of units								71,300,000			69,000,000	71,300,000		69,000,000
Common Units | Subsequent Events
Distributions
Per Unit Distribution, declared (in dollars per unit)				$ 0.65
Limited partners, Distribution declared	46.0
Common Units | Limited Partners
Distributions
Partnership distribution												$ 117.0
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] OwnershipEquity EarningsEquity InvestmentsInterest atThree monthsSix Months(unaudited)June 30, ended June 30, ended June 30, June 30, December 31, (millions of dollars)2018    20182017    2018201720182017Northern Border (a) 50% 15  15 32  34 505 512Great Lakes 46.45% 12 6 36 23 485 479Iroquois(b) 49.34% 9  3 27  3 221 222  36  24 95  60 1,211 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.Equity earnings from Iroquois is net of the 29-year amortization of a $10 million purchase price discrepancy assumed by the Partnership from TransCanada at the time of the 2017 Acquisition.