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PARTNERS' EQUITY - ATM Equity Issuance Program (Details)
shares in Millions, $ in Millions
2 Months Ended 12 Months Ended
Aug. 05, 2016
USD ($)
item
May 19, 2016
shares
Dec. 31, 2018
USD ($)
shares
Dec. 31, 2017
USD ($)
shares
Dec. 31, 2016
USD ($)
shares
Aug. 31, 2014
USD ($)
TC PipeLines GP, Inc. | General Partner            
PARTNERS' EQUITY            
Ownership interest in the Partnership (as a percent)     2.00% 2.00% 2.00%  
ATM Equity Issuance Program | Common Units            
PARTNERS' EQUITY            
Aggregate offering price of units           $ 200.0
Units sold | shares     0.7 3.2 3.1  
Net proceeds from issuance of common units     $ 39.0 $ 173.0 $ 164.0  
Sales agent commissions     0.0 2.0 2.0  
Reclassification of common unit issuance subject to rescission, net (in units) | shares   1.6        
Common units subject to rescission     0.0      
ATM Equity Issuance Program | TC PipeLines GP, Inc. | General Partner            
PARTNERS' EQUITY            
Equity contribution     $ 1.0 $ 3.0 $ 3.0  
Equity Distribution Agreement (EDA) | Common Units            
PARTNERS' EQUITY            
Amended shelf registration with SEC $ 400.0          
Number of financial institutions | item 5