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PARTNERS' EQUITY - Ownership (Details) - shares
12 Months Ended
Apr. 01, 2015
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Common Units        
PARTNERS' EQUITY        
Common units outstanding, end of year (in units)   70,600,000 67,400,000 [1],[2] 64,300,000 [1]
Common Units | Limited Partners        
PARTNERS' EQUITY        
Common units outstanding, end of year (in units)   70,573,423    
Non-affiliates | Common Units | Limited Partners        
PARTNERS' EQUITY        
Common units outstanding, end of year (in units)   53,488,592    
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)   24.20%    
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%    
[1] Recast to consolidate PNGTS (Refer to Notes 2 and 7).
[2] Recast to consolidate PNGTS (Refer to Notes 2 and 7).