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ORGANIZATION - Narrative (Details) - shares
12 Months Ended
Dec. 14, 2020
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
TC Energy Merger        
Partners' Equity        
Common shares exchanged in merger (in shares) 0.70      
Common units        
Partners' Equity        
Number of units (in units)   71,300,000 71,300,000 71,300,000
Common units | General Partner | TC Pipelines, LP        
Partners' Equity        
Limited partner interest (as a percent)   2.00%    
Class B units | TC Energy | TC Energy        
Partners' Equity        
Limited partner interest (as a percent)   24.00%    
Limited Partners | Common units        
Partners' Equity        
Number of units (in units)   71,306,396    
Limited Partners | Common units | TC Pipelines, LP        
Partners' Equity        
Number of units (in units)   5,797,106    
Limited Partners | Common units | TC Energy        
Partners' Equity        
Number of units (in units)   11,287,725    
Limited Partners | Class B units | TC Pipelines, LP        
Partners' Equity        
Limited partner interest (as a percent)   100.00%    
Limited Partners | Class B units | TC Energy        
Partners' Equity        
Number of units (in units)   1,900,000    
Limited partner interest (as a percent)   100.00%    
General Partner | TC Pipelines, LP        
Partners' Equity        
IDRs ownership   100.00%