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PARTNERS' EQUITY - May 2013 Equity Issuance (Details 4) - USD ($)
$ / shares in Units, $ in Millions
12 Months Ended
Apr. 01, 2015
May. 22, 2013
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Partners' Equity          
Equity contribution     $ 2    
General Partner          
Partners' Equity          
Equity contribution $ 2   $ 2    
TC PipeLines GP, Inc. | General Partner          
Partners' Equity          
Ownership interest in the Partnership (as a percent) 2.00% 2.00% 2.00% 2.00% 2.00%
Equity Issuance, May 2013 | TC PipeLines GP, Inc. | General Partner          
Partners' Equity          
Equity contribution   $ 8      
Equity Issuance, May 2013 | Common units          
Partners' Equity          
Units sold   8,855,000      
Units sold in public offering pursuant to exercise of underwriters' option   1,155,000      
Price per common unit in public offering   $ 43.85      
Gross proceeds from sale of common units   $ 388      
Net proceeds from public offering of common units   $ 373