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SIGNIFICANT ACCOUNTING POLICIES - Ownership Interests Acquired (Details)
Aug. 01, 2017
Jun. 01, 2017
Jan. 01, 2016
Apr. 01, 2015
Mar. 31, 2015
Acquisitions          
Ownership interest (as a percent) 49.34%        
Portland Natural Gas Transmission System          
Acquisitions          
Ownership interest, including acquired interest (as a percent)   61.71%      
Ownership interest (as a percent)   11.81% 49.90%    
Interest acquired (as a percent)   61.71%      
GTN          
Acquisitions          
Ownership interest (as a percent)       30.00%  
Iroquois          
Acquisitions          
Interest acquired (as a percent)   49.34%      
Former parent, TransCanada subsidiaries | Portland Natural Gas Transmission System | Transaction between entities under common control          
Acquisitions          
Interest acquired (as a percent)     49.90%    
TransCanada | GTN          
Acquisitions          
Remaining noncontrolling ownership interest (as a percent)         30.00%
GTN | Former parent, TransCanada subsidiaries | Transaction between entities under common control          
Acquisitions          
Interest acquired (as a percent)       30.00%  
Portland Natural Gas Transmission System          
Acquisitions          
Ownership interest, including acquired interest (as a percent)   61.71%      
Portland Natural Gas Transmission System | Former parent, TransCanada subsidiaries | Transaction between entities under common control          
Acquisitions          
Ownership interest (as a percent)     49.90%