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ORGANIZATION - Ownership Interests in Natural Gas Pipeline Systems (Details)
12 Months Ended
Dec. 31, 2016
LimitedPartnership
mi
Jan. 01, 2016
Dec. 31, 2015
Mar. 31, 2015
Sep. 30, 2014
Organization          
Number of intermediate limited partnerships through which pipeline assets are owned | LimitedPartnership 3        
Northern Border          
Organization          
Remaining ownership interest (as a percent) 50.00%        
Interest acquired (as a percent) 50.00%        
Great Lakes          
Organization          
Interest acquired (as a percent) 46.45%   46.45% 46.45%  
Portland Natural Gas Transmission System          
Organization          
Interest acquired (as a percent) 49.90% 49.90%      
TransCanada | GTN          
Organization          
Remaining noncontrolling ownership interest (as a percent)       30.00%  
TransCanada | Bison          
Organization          
Remaining noncontrolling ownership interest (as a percent)         30.00%
TransCanada | Portland Natural Gas Transmission System          
Organization          
Remaining noncontrolling ownership interest (as a percent) 11.81%        
North 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        
Remaining 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%        
Bison          
Organization          
Length of pipeline owned (in miles) 303        
Remaining ownership interest (as a percent) 100.00%        
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%        
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%        
Portland Natural Gas Transmission System          
Organization          
Length of pipeline owned (in miles) 295        
Ownership interest (as a percent) 49.90%        
TransCanada | Great Lakes          
Organization          
Remaining ownership interest (as a percent) 53.55%        
ONEOK Partners, L.P. | Northern Border          
Organization          
Remaining ownership interest (as a percent) 50.00%