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CASH DISTRIBUTIONS - Distributions Paid (Details) - USD ($)
$ / shares in Units, $ in Millions
3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Common Units    
Partners' Equity    
Per Unit Distribution, paid (in dollars per unit) $ 1.00 $ 0.94
Total cash distribution $ 76 $ 68
Common units and General Partner interest combined    
Partners' Equity    
Total distribution of general partner interest and IDR payment 5  
TC PipeLines GP, Inc. | General Partner    
Partners' Equity    
Total distribution for General Partner interest $ 2 $ 2
General partner interest (as a percent) 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 $ 2