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SUBSEQUENT EVENTS (Details) - USD ($) $ / shares in Units, $ in Millions									3 Months Ended		6 Months Ended
	Sep. 29, 2020	Aug. 04, 2020	Aug. 01, 2020	Jul. 31, 2020	Jul. 23, 2020	Jul. 15, 2020	Jul. 09, 2020	Jul. 08, 2020	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Distributions
Cash distribution											$ 8	$ 13
Partnership's share of distributions											$ 115	$ 112
Northern Border
Distributions
Ownership interest									50.00%		50.00%
Great Lakes
Distributions
Ownership interest									46.45%	46.45%	46.45%	46.45%
General Partner \| TC Pipelines, LP \| General Partner
Distributions
General partner interest									2.00%	2.00%	2.00%	2.00%
Subsequent Events
Distributions
Cash distribution					$ 47
Subsequent Events \| Natural gas transmission IT application \| ANR Pipeline Company
Distributions
Intangible assets acquired			$ 51
Subsequent Events \| Two Percent interest
Distributions
Cash distribution, general partner					$ 1
Subsequent Events \| Northern Border
Distributions
Ownership interest				50.00%
Partnership's share of distributions				$ 5
Subsequent Events \| Great Lakes
Distributions
Ownership interest				46.45%
Partnership's share of distributions				$ 11
Subsequent Events \| General Partner \| TC Pipelines, LP \| General Partner
Distributions
General partner interest					2.00%
Subsequent Events \| Northern Border
Distributions
Limited partners, distribution declared							$ 11
Subsequent Events \| Great Lakes
Distributions
Limited partners, distribution declared						$ 24
Subsequent Events \| Iroquois
Distributions
Limited partners, distribution declared		$ 21
Subsequent Events \| PNGTS
Distributions
Limited partners, distribution declared								$ 14
Share of distributions to its non-controlling interest owner				$ 5
Subsequent Events \| TC Pipelines, LP \| Natural gas transmission IT application \| ANR Pipeline Company
Distributions
Intangible assets acquired			$ 38
Common Units \| Subsequent Events
Distributions
Distribution declared (in dollars per unit)					$ 0.65
Cash distribution					$ 46
Common Units \| Subsequent Events \| General Partner
Distributions
Cash distribution					$ 4
Number of units held by investor					5,797,106
Common Units \| Subsequent Events \| TC Energy
Distributions
Cash distribution					$ 7
Number of units held by investor					11,287,725
Forecast \| Iroquois
Distributions
Ownership interest	49.34%
Partnership's share of distributions	$ 10