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SUBSEQUENT EVENTS - Distributions (Details) - USD ($)																						1 Months Ended	3 Months Ended	12 Months Ended
SUBSEQUENT EVENTS - Distributions (Details) - USD ($)	Mar. 24, 2021	Feb. 26, 2021	Feb. 18, 2021	Feb. 16, 2021	Feb. 12, 2021	Jan. 29, 2021	Jan. 19, 2021	Jan. 15, 2021	Jan. 13, 2021	Nov. 13, 2020	Aug. 14, 2020	May 12, 2020	Feb. 14, 2020	Nov. 14, 2019	Aug. 14, 2019	May 13, 2019	Feb. 11, 2019	Nov. 14, 2018	Aug. 15, 2018	May 15, 2018	Feb. 13, 2018	Jun. 30, 2019	Dec. 31, 2020	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Distributions
Cash distribution																								$ 8,000,000	$ 13,000,000	$ 15,000,000
Partnership's share of distributions																								196,000,000	200,000,000	188,000,000
TC PipeLines, LP Senior Credit Facility due 2021
Distributions
Debt and credit facilities																							$ 0	0	0
Maximum borrowing capacity																							500,000,000	500,000,000
TC PipeLines, LP Senior Credit Facility due 2021 \| Revolving Credit Facility
Distributions
Maximum borrowing capacity																							500,000,000	500,000,000
TC PipeLines, LP 4.65% Senior Notes due 2021
Distributions
Debt and credit facilities																							$ 350,000,000	$ 350,000,000	$ 350,000,000
Stated interest rate																							4.65%	4.65%	4.65%
TC Pipelines, LP
Distributions
Ownership interest in the partnership																								2.00%
Northern Border
Distributions
Ownership interest																							50.00%	50.00%
Great Lakes
Distributions
Ownership interest																							46.45%	46.45%
Equity contribution																								$ 10,000,000	$ 10,000,000	$ 9,000,000
General Partner
Distributions
Total cash distribution																							$ 0
Northern Border
Distributions
Cash distribution																						$ 100,000,000
Northern Border \| Revolving Credit Facility
Distributions
Maximum borrowing capacity																						$ 200,000,000
Subsequent Events
Distributions
Total cash distribution					$ 47,000,000
Subsequent Events \| TC PipeLines, LP 4.65% Senior Notes due 2021
Distributions
Redemption price					100.00%
Subsequent Events \| Northern Border
Distributions
Ownership interest				50.00%				50.00%
Partnership's share of distributions		$ 9,000,000				$ 8,000,000
Subsequent Events \| Great Lakes
Distributions
Ownership interest									46.45%
Partnership's share of distributions						11,000,000
Subsequent Events \| Iroquois
Distributions
Ownership interest			49.34%
Partnership's share of distributions	$ 11,000,000
Subsequent Events \| Northern Border
Distributions
Cash distribution								$ 16,000,000
Limited partners, Distribution declared				$ 18,000,000
Subsequent Events \| Great Lakes
Distributions
Limited partners, Distribution declared									$ 23,000,000
Subsequent Events \| Iroquois
Distributions
Limited partners, Distribution declared			$ 22,000,000
Subsequent Events \| Iroquois \| Forecast
Distributions
Equity contribution	$ 1,000,000
Subsequent Events \| PNGTS
Distributions
Limited partners, Distribution declared									$ 12,000,000
Share of distributions to its non-controlling interest owner						$ 5,000,000
TC Pipelines, LP \| Subsequent Events \| Two Percent interest
Distributions
General partnership distribution paid					$ 1,000,000
Common units
Distributions
Cash distribution										$ 46,000,000	$ 46,000,000	$ 46,000,000	$ 46,000,000	$ 46,000,000	$ 46,000,000	$ 46,000,000	$ 46,000,000	$ 46,000,000	$ 46,000,000	$ 46,000,000	$ 71,000,000
Number of units (in units)																							71,300,000	71,300,000	71,300,000	71,300,000
Common units \| General Partner \| TC Pipelines, LP
Distributions
Ownership interest in the partnership																								2.00%	2.00%	2.00%
Common units \| Subsequent Events
Distributions
Per Unit Distribution, declared (in dollars per unit)							$ 0.65
Cash distribution					46,000,000
Common units \| Limited Partners
Distributions
Number of units (in units)																							71,306,396	71,306,396
Common units \| TC Pipelines, LP \| Subsequent Events
Distributions
Cash distribution					$ 4,000,000
Common units \| TC Pipelines, LP \| Limited Partners
Distributions
Number of units (in units)																							5,797,106	5,797,106
Common units \| TC Pipelines, LP \| Limited Partners \| Subsequent Events
Distributions
Number of units (in units)					5,797,106
Common units \| TC Energy \| Subsequent Events
Distributions
Cash distribution					$ 7,000,000
Common units \| TC Energy \| Limited Partners
Distributions
Number of units (in units)																							11,287,725	11,287,725
Common units \| TC Energy \| Limited Partners \| Subsequent Events
Distributions
Number of units (in units)					11,287,725