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CASH DISTRIBUTIONS - Distributions Paid (Details) - USD ($)
$ / shares in Units, $ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2017
Sep. 30, 2016
Sep. 30, 2017
Sep. 30, 2016
Common Units        
Distributions        
Per Unit Distribution, paid (in dollars per unit) $ 1.00 $ 0.94 $ 2.88 $ 2.72
Total cash distribution $ 74 $ 65 $ 210 $ 184
Common units and General Partner interest combined        
Distributions        
Total distribution of general partner interest and IDR payment 5   11  
TC PipeLines GP, Inc. | General Partner        
Distributions        
Total distribution for General Partner interest $ 2 $ 1 $ 4 $ 3
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        
Distributions        
Incentive distribution paid to the General Partner $ 3 $ 2 $ 7 $ 4