XML 67 R53.htm IDEA: XBRL DOCUMENT v3.7.0.1
SIGNIFICANT ACCOUNTING POLICIES - Ownership Interests Acquired (Details)
Aug. 01, 2017
Jun. 01, 2017
Dec. 31, 2016
Jan. 31, 2016
Jan. 01, 2016
Dec. 31, 2015
Apr. 01, 2015
Mar. 31, 2015
Oct. 01, 2014
Sep. 30, 2014
Portland Natural Gas Transmission System                    
Acquisitions                    
Ownership interest (as a percent)       49.90%            
Iroquois                    
Acquisitions                    
Ownership interest (as a percent) 49.34% 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 | Portland Natural Gas Transmission System                    
Acquisitions                    
Remaining noncontrolling ownership interest (as a percent)     38.29%              
TransCanada | GTN                    
Acquisitions                    
Remaining noncontrolling ownership interest (as a percent)               30.00%    
TransCanada | Bison                    
Acquisitions                    
Remaining noncontrolling ownership interest (as a percent)                   30.00%
Portland Natural Gas Transmission System                    
Acquisitions                    
Ownership interest (as a percent)   11.81%     49.90%          
Interest acquired (as a percent)   61.71%                
Portland Natural Gas Transmission System | Former parent, TransCanada subsidiaries | Transaction between entities under common control                    
Acquisitions                    
Interest acquired (as a percent)     30.00%   49.90% 49.90%        
GTN | Former parent, TransCanada subsidiaries | Transaction between entities under common control                    
Acquisitions                    
Interest acquired (as a percent)             30.00%      
Bison | Former parent, TransCanada subsidiaries | Transaction between entities under common control                    
Acquisitions                    
Interest acquired (as a percent)     30.00%       30.00%   30.00%  
Subsequent event | Portland Natural Gas Transmission System                    
Acquisitions                    
Interest acquired (as a percent)   11.81%                
Subsequent event | Iroquois                    
Acquisitions                    
Ownership interest (as a percent) 49.34%                  
Subsequent event | Portland Natural Gas Transmission System                    
Acquisitions                    
Ownership interest, including acquired interest (as a percent)   61.71%                
Subsequent event | Portland Natural Gas Transmission System | Transaction between entities under common control                    
Acquisitions                    
Ownership interest, including acquired interest (as a percent)   61.71%