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ORGANIZATION - Capitalization (Details) - shares
12 Months Ended
Apr. 01, 2015
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Common Units        
Partners' Equity        
Number of units   70,600,000 67,400,000 [1],[2] 64,300,000 [1]
General Partner | TC PipeLines GP, Inc.        
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%
Limited Partners | Common Units        
Partners' Equity        
Number of units   70,573,423    
Limited Partners | Common Units | TC PipeLines GP, Inc.        
Partners' Equity        
Number of units   5,797,106    
Limited Partners | Common Units | TransCanada        
Partners' Equity        
Number of units   11,287,725    
Limited partner interest (as a percent)   24.20%    
Limited Partners | Class B Units | TransCanada        
Partners' Equity        
Number of units   1,900,000    
Limited partner interest (as a percent)   100.00%    
[1] Recast to consolidate PNGTS (Refer to Notes 2 and 7).
[2] Recast to consolidate PNGTS (Refer to Notes 2 and 7).