CASH DISTRIBUTIONS - Quarterly Distributions (Details)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Cash distributions
Period after the end of each quarter within which quarterly cash distributions to partners are to be paid	45 days
TC Pipelines, LP
Cash distributions
Ownership interest in the partnership	2.00%
General Partner | TC Pipelines, LP | Common units
Cash distributions
Ownership interest in the partnership	2.00%	2.00%	2.00%