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ORGANIZATION - Ownership Interests in Natural Gas Pipeline Systems (Details)
12 Months Ended
Oct. 31, 2019
Dec. 31, 2019
mi
Dec. 31, 2018
LimitedPartnership
Jan. 06, 2020
Aug. 01, 2017
Jun. 01, 2017
Organization            
Number of intermediate limited partnerships through which pipeline assets are owned | LimitedPartnership     3      
PNGTS            
Organization            
Ownership interest (as a percent)           11.81%
Northern Border            
Organization            
Ownership interest (as a percent)   50.00%        
Great Lakes            
Organization            
Ownership interest (as a percent)   46.45%        
Iroquois            
Organization            
Ownership interest (as a percent)   49.34%   49.34% 49.34% 49.34%
GTN            
Organization            
Length of pipeline owned (in miles)   1,377        
Ownership interest (as a percent) 100.00% 100.00%        
Bison            
Organization            
Length of pipeline owned (in miles)   303        
Ownership interest (as a percent)   100.00%        
North Baja            
Organization            
Length of pipeline owned (in miles)   86        
Ownership interest (as a percent)   100.00%        
Tuscarora            
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%        
Northern Border | Northern Border            
Organization            
Ownership interest (as a percent)   50.00%        
Northern Border | ONEOK Partners, L.P.            
Organization            
Remaining ownership interest (as a percent)   50.00%        
PNGTS            
Organization            
Length of pipeline owned (in miles)   295        
Ownership interest (as a percent)   61.71%        
PNGTS | Maritimes and Northeast Pipeline LLC            
Organization            
Length of pipeline owned (in miles)   107        
PNGTS | Maritimes and Northeast Pipeline LLC | PNGTS            
Organization            
Ownership interest (as a percent)   32.00%        
Great Lakes            
Organization            
Length of pipeline owned (in miles)   2,115        
Ownership interest (as a percent)   46.45%        
Great Lakes | TC Energy            
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%        
Iroquois Gas | TC Energy            
Organization            
Ownership interest (as a percent)   0.66%        
Iroquois Gas | TC Energy            
Organization            
Remaining ownership interest (as a percent)   50.66%        
Northern New England Investment | PNGTS            
Organization            
Remaining noncontrolling ownership interest (as a percent)   38.29%