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CASH DISTRIBUTIONS TO COMMON UNITS - Distributions Paid (Details) - USD ($)
$ / shares in Units, $ in Millions
3 Months Ended 9 Months Ended 12 Months Ended
Feb. 14, 2017
Nov. 14, 2016
Aug. 12, 2016
May 13, 2016
Feb. 12, 2016
Nov. 13, 2015
Aug. 14, 2015
May 15, 2015
Apr. 01, 2015
Feb. 13, 2015
Nov. 14, 2014
Aug. 14, 2014
May 15, 2014
Feb. 14, 2014
Mar. 31, 2017
Mar. 31, 2016
Sep. 30, 2016
Sep. 30, 2015
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Distributions                                          
Incentive distribution paid to the General Partner $ 2 $ 2 $ 2 $ 1 $ 1 $ 1 $ 1       $ 1               $ 6 $ 2 $ 1
Common units                                          
Distributions                                          
Per Unit Distribution, paid (in dollars per unit) $ 0.94 $ 0.94 $ 0.94 $ 0.89 $ 0.89 $ 0.89 $ 0.89 $ 0.84   $ 0.84 $ 0.84 $ 0.84 $ 0.81 $ 0.81 $ 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% 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                             $ 2 $ 1