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CASH DISTRIBUTIONS - Distributions Paid (Details) - USD ($)
$ / shares in Units, $ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Common Units        
Partners' Equity        
Per Unit Distribution, paid (in dollars per unit) $ 0.65 $ 1.00 $ 2.30 $ 2.88
Total cash distribution $ 47 $ 74 $ 171 $ 210
Common units and General Partner interest combined        
Partners' Equity        
Total distribution of general partner interest and IDR payment     7  
TC PipeLines GP, Inc. | General Partner        
Partners' Equity        
Total distribution for General Partner interest $ 1 $ 2 $ 4 $ 4
General partner interest (as a percent) 2.00% 2.00% 2.00% 2.00%
TC PipeLines GP, Inc. | Common units and General Partner interest combined        
Partners' Equity        
Incentive distribution paid to the General Partner   $ 3 $ 3 $ 7