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PARTNERS' EQUITY - ATM Equity Issuance Program (Details) - USD ($)
shares in Millions, $ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
PARTNERS' EQUITY        
Net proceeds from issuance of common units     $ 40  
General Partner        
PARTNERS' EQUITY        
Net proceeds from issuance of common units     $ 1  
TC PipeLines GP, Inc. | General Partner        
PARTNERS' EQUITY        
Ownership interest in the Partnership (as a percent) 2.00% 2.00% 2.00% 2.00%
ATM Equity Issuance Program | Common Units        
PARTNERS' EQUITY        
Units sold 0.0   0.7  
Sales agent commissions     $ 0  
Net proceeds from issuance of common units     39  
ATM Equity Issuance Program | TC PipeLines GP, Inc. | General Partner        
PARTNERS' EQUITY        
Equity contribution     $ 1