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ORGANIZATION - Ownership Interests in Natural Gas Pipeline Systems (Details)
12 Months Ended
Dec. 31, 2017
LimitedPartnership
mi
Feb. 28, 2018
Jan. 31, 2018
Sep. 01, 2017
Aug. 01, 2017
Jun. 01, 2017
Dec. 31, 2016
Jan. 01, 2016
Dec. 31, 2015
Organization                  
Number of intermediate limited partnerships through which pipeline assets are owned | LimitedPartnership 3                
Interest acquired (as a percent)         49.34%        
Northern Border                  
Organization                  
Interest acquired (as a percent) 50.00% 50.00% 50.00%            
Portland Natural Gas Transmission System                  
Organization                  
Interest acquired (as a percent)           11.81%   49.90%  
Interest acquired by Partnership (as a percent)           61.71%      
Ownership interest, including acquired interest (as a percent)           61.71%      
Northern New England Investment | Portland Natural Gas Transmission System                  
Organization                  
Remaining noncontrolling ownership interest (as a percent) 38.29%                
GTN                  
Organization                  
Length of pipeline owned (in miles) 1,377                
Ownership interest (as a percent) 100.00%                
Bison                  
Organization                  
Length of pipeline owned (in miles) 303                
Ownership interest (as a percent) 100.00%                
North Baja Pipeline, LLC                  
Organization                  
Length of pipeline owned (in miles) 86                
Ownership interest (as a percent) 100.00%                
Tuscarora Gas Transmission Company                  
Organization                  
Length of pipeline owned (in miles) 305                
Ownership interest (as a percent) 100.00%                
Northern Border                  
Organization                  
Length of pipeline owned (in miles) 1,412                
Ownership interest (as a percent) 50.00%                
Interest acquired (as a percent)       50.00%          
Portland Natural Gas Transmission System                  
Organization                  
Length of pipeline owned (in miles) 295                
Interest acquired (as a percent) 61.71%                
Interest acquired by Partnership (as a percent) 61.71%           61.71%   61.71%
Portland Natural Gas Transmission System | Maritimes and Northeast Pipeline LLC                  
Organization                  
Length of pipeline owned (in miles) 107                
Great Lakes                  
Organization                  
Length of pipeline owned (in miles) 2,115                
Ownership interest (as a percent) 46.45%                
Great Lakes | TransCanada                  
Organization                  
Remaining noncontrolling ownership interest (as a percent) 53.55%                
Iroquois Gas                  
Organization                  
Length of pipeline owned (in miles) 416                
Ownership interest (as a percent) 49.34%                
Remaining ownership interest (as a percent) 50.66%                
Iroquois Gas | TransCanada                  
Organization                  
Remaining ownership interest (as a percent) 0.66%                
Iroquois Gas | Dominion Midstream                  
Organization                  
Remaining ownership interest (as a percent) 25.93%                
Iroquois Gas | Dominion Resources                  
Organization                  
Remaining ownership interest (as a percent) 24.07%                
ONEOK Partners, L.P. | Northern Border                  
Organization                  
Remaining ownership interest (as a percent) 50.00%                
Interest acquired (as a percent) 50.00%                
Portland Natural Gas Transmission System                  
Organization                  
Ownership interest, including acquired interest (as a percent)           61.71%      
Portland Natural Gas Transmission System | Maritimes and Northeast Pipeline LLC                  
Organization                  
Interest acquired (as a percent) 32.00%