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PARTNERS' EQUITY - ATM Equity Issuance Program (Details) - USD ($)
3 Months Ended 6 Months Ended
Jun. 30, 2017
Jun. 30, 2016
Jun. 30, 2017
Jun. 30, 2016
PARTNERS' EQUITY        
Net proceeds from issuance of common units [1]     $ 92,000,000  
General Partner        
PARTNERS' EQUITY        
Net proceeds from issuance of common units     $ 2,000,000  
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     1,542,921  
Net proceeds from issuance of common units     $ 90,000,000  
Sales agent commissions     1  
ATM Equity Issuance Program | TC PipeLines GP, Inc. | General Partner        
PARTNERS' EQUITY        
Equity contribution     $ 2,000,000  
[1] Recast to consolidate PNGTS for all periods presented. See Notes 2 and 6.