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ORGANIZATION - Capitalization (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                  
Number of units [1]   68,600,000     67,400,000 64,300,000 63,600,000   64,700,000
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% 2.00% 2.00% 2.00%    
Limited Partners | Common units                  
Partners' Equity                  
Number of units         67,454,831        
Limited Partners | Common units | TC PipeLines GP, Inc.                  
Partners' Equity                  
Number of units         5,797,106     5,797,106  
Limited Partners | Common units | TransCanada                  
Partners' Equity                  
Number of units         11,287,725        
Limited partner interest (as a percent)         25.30%        
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 for all periods presented (Refer to Note 2).