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PARTNERS' EQUITY - Ownership (Details) - shares
3 Months Ended 9 Months Ended 12 Months Ended
Apr. 01, 2015
Mar. 31, 2017
Sep. 30, 2016
Sep. 30, 2015
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Apr. 25, 2017
Mar. 31, 2016
Common units                  
Partners' Equity                  
Common units outstanding, end of year (in units) [1]   68,600,000     67,400,000 64,300,000 63,600,000   64,700,000
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% 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     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%        
[1] Recast to consolidate PNGTS for all periods presented (Refer to Note 2).