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PARTNERS' EQUITY - Ownership (Details) - shares
12 Months Ended
Apr. 01, 2015
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Common units        
Partners' Equity        
Common units outstanding, end of year (in units)   67,400,000 64,300,000 63,600,000
Common units | Limited Partners        
Partners' Equity        
Common units outstanding, end of year (in units)   67,454,831    
Non-affiliates | Common units | Limited Partners        
Partners' Equity        
Common units outstanding, end of year (in units)   50,370,000    
TC PipeLines GP, Inc. | General Partner        
Partners' Equity        
IDRs ownership (as a percent)   100.00%    
Ownership interest in the Partnership (as a percent) 2.00% 2.00% 2.00% 2.00%
TC PipeLines GP, Inc. | Common units | Limited Partners        
Partners' Equity        
Common units outstanding, end of year (in units)   5,797,106    
TransCanada Corporation and subsidiaries | Common units | Limited Partners        
Partners' Equity        
Common units outstanding, end of year (in units)   17,084,831    
TransCanada | Common units | Limited Partners        
Partners' Equity        
Common units outstanding, end of year (in units)   11,287,725    
Ownership interest in the Partnership (as a percent)   25.30%    
TransCanada | Class B units | Limited Partners        
Partners' Equity        
Common units outstanding, end of year (in units)   1,900,000    
Ownership interest in the Partnership (as a percent)   100.00%