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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 ($)
item
Apr. 01, 2015
USD ($)
May 19, 2016
shares
Dec. 31, 2017
USD ($)
shares
Dec. 31, 2016
USD ($)
shares
Dec. 31, 2015
USD ($)
shares
Aug. 31, 2014
USD ($)
PARTNERS' EQUITY              
Equity contribution [1]           $ 2.0  
Common units subject to rescission [2],[3]         $ 83.0    
General Partner              
PARTNERS' EQUITY              
Equity contribution   $ 2.0       $ 2.0  
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              
PARTNERS' EQUITY              
Net proceeds from issuance of common units [1]       $ 176.0 $ 84.0 $ 44.0  
ATM Equity Issuance Program | General Partner              
PARTNERS' EQUITY              
Net proceeds from issuance of common units       $ 3.0 $ 2.0 $ 1.0  
ATM Equity Issuance Program | Common Units              
PARTNERS' EQUITY              
Aggregate offering price of units             $ 200.0
Units sold | shares       3.2 3.1 0.7  
Net proceeds from issuance of common units       $ 173.0 $ 164.0 $ 43.0  
Sales agent commissions       2.0 2.0 0.4  
Reclassification of common unit issuance subject to rescission, net (in units) | shares     1.6        
Common units subject to rescission       0.0 $ 83.0    
Common units | shares         1.6    
ATM Equity Issuance Program | TC PipeLines GP, Inc. | General Partner              
PARTNERS' EQUITY              
Equity contribution       $ 3.0 $ 3.0 $ 1.0  
Equity Distribution Agreement (EDA) | Common Units              
PARTNERS' EQUITY              
Amended shelf registration with SEC $ 400.0            
Number of financial institutions | item 5            
[1] Recast to consolidate PNGTS (Refer to Notes 2 and 7).
[2] Recast to consolidate PNGTS (Refer to Notes 2 and 7).
[3] Recast to consolidate PNGTS (Refer to Notes 2 and 7).