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The contractual term of long term transportation capacity.

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Amount of cash distribution paid to unit-holder of limited partnership (LP).

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Per unit of ownership amount of cash distributions declared to unit-holder of a limited partnership (LP).

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Amount of distribution received from equity method investee for return on investment, classified as operating activities. Excludes distribution for return of investment, classified as investing activities.

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SUBSEQUENT EVENTS (Details) $ / shares in Units, MMBTU / d in Thousands, $ in Millions									3 Months Ended		9 Months Ended
	Dec. 30, 2020 USD ($)	Nov. 04, 2020 USD ($)	Oct. 30, 2020 USD ($)	Oct. 21, 2020 USD ($) $ / shares shares	Oct. 09, 2020 USD ($)	Oct. 08, 2020 USD ($)	Oct. 05, 2020 tradingDay $ / shares	Nov. 01, 2017 Bcf	Sep. 30, 2020 USD ($) MMBTU / d	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($) MMBTU / d	Sep. 30, 2019 USD ($)
Subsequent Event [Line Items]
Cash distribution											$ 8	$ 13
Partnership's share of distributions											$ 161	168
Northern Border
Subsequent Event [Line Items]
Ownership interest									50.00%		50.00%
Partnership's share of distributions									$ 22	$ 21	$ 68	$ 121
General Partner \| TC Pipelines, LP \| General Partner
Subsequent Event [Line Items]
General partner interest									2.00%	2.00%	2.00%	2.00%
Great Lakes \| TC Energy \| Transportation contracts
Subsequent Event [Line Items]
Contract term								10 years
Transportation capacity per day \| Bcf								0.711
Contract capacity \| MMBTU / d									722		722
Subsequent events
Subsequent Event [Line Items]
Cash distribution declared				$ 47
Subsequent events \| Two Percent interest
Subsequent Event [Line Items]
Cash distribution, general partner				$ 1
Subsequent events \| Northern Border
Subsequent Event [Line Items]
Ownership interest			50.00%
Partnership's share of distributions			$ 9
Subsequent events \| General Partner \| TC Pipelines, LP \| General Partner
Subsequent Event [Line Items]
General partner interest				2.00%
Subsequent events \| Northern Border
Subsequent Event [Line Items]
Limited partners, distribution declared					$ 18
Subsequent events \| Iroquois
Subsequent Event [Line Items]
Limited partners, distribution declared		$ 21
Subsequent events \| PNGTS
Subsequent Event [Line Items]
Limited partners, distribution declared						$ 14
Share of distributions to its non-controlling interest owner			$ 5
Common Units \| Subsequent events
Subsequent Event [Line Items]
Distribution declared (in dollars per share) \| $ / shares				$ 0.65
Cash distribution declared				$ 46
Common Units \| Subsequent events \| Share distribution
Subsequent Event [Line Items]
Conversion ratio							0.65
Implied value of distribution (in dollars per share) \| $ / shares							$ 27.31
Implied premium							7.50%
Number of trading days for weighted average prices \| tradingDay							20
Common Units \| Subsequent events \| General Partner
Subsequent Event [Line Items]
Cash distribution declared				$ 4
Number of units held by investor (in shares) \| shares				5,797,106
Common Units \| Subsequent events \| TC Energy
Subsequent Event [Line Items]
Cash distribution declared				$ 7
Number of units held by investor (in shares) \| shares				11,287,725
Forecast \| Iroquois
Subsequent Event [Line Items]
Ownership interest	49.34%
Partnership's share of distributions	$ 10