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PARTNERS' EQUITY - ATM Equity Issuance Program (Details) - USD ($)
shares in Millions, $ in Millions
3 Months Ended
Mar. 31, 2018
Mar. 31, 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%
ATM Equity Issuance Program | Common Units    
PARTNERS' EQUITY    
Units sold 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