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CASH DISTRIBUTIONS TO COMMON UNITS - Distributions Paid (Details) - USD ($)
$ / shares in Units, $ in Millions
3 Months Ended 9 Months Ended
Mar. 31, 2017
Mar. 31, 2016
Sep. 30, 2016
Sep. 30, 2015
Common units        
Distributions        
Per Unit Distribution, paid (in dollars per unit) $ 0.94 $ 0.89    
Total cash distribution $ 68 $ 60    
Common units and General Partner interest combined        
Distributions        
Total distribution of general partner interest and IDR payment 4      
TC PipeLines GP, Inc. | General Partner        
Distributions        
Total distribution for General Partner interest $ 2 1    
General partner interest (as a percent) 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 $ 2 $ 1