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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%