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CASH DISTRIBUTIONS PAID TO COMMON UNITS (Details) - USD ($)
$ / shares in Units, $ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2019
Sep. 30, 2018
Sep. 30, 2019
Sep. 30, 2018
Common Units        
Partners' Equity        
Per Unit Distribution, paid (in dollars per unit) $ 0.65 $ 0.65 $ 1.95 $ 2.30
Total cash distribution $ 47 $ 47 $ 142 $ 171
General Partner | TC Pipelines, LP        
Partners' Equity        
Total distribution for General Partner interest   1   4
General Partner | General Partner        
Partners' Equity        
Total distribution for General Partner interest $ 1 1 $ 3 7
General Partner | General Partner | Incentive Distribution Rights        
Partners' Equity        
Incentive distribution paid to the General Partner   $ 0   $ 3
General Partner | General Partner | TC Pipelines, LP        
Partners' Equity        
General partner interest (as a percent) 2.00% 2.00% 2.00% 2.00%