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PARTNERS' EQUITY - Class B Units (Details) - Class B Units - USD ($)
$ in Millions
3 Months Ended 6 Months Ended
Mar. 31, 2018
Jun. 30, 2018
Jun. 30, 2017
PARTNERS' EQUITY      
Limited Partners, Distributions paid   $ 15 $ 22
Distributions      
PARTNERS' EQUITY      
Limited Partners, Distributions paid $ 15    
GTN | TransCanada | Distributions      
PARTNERS' EQUITY      
Portion of GTN's annual distributions to which the Class B units are entitled to receive a percentage share of the distributions above threshold (as a percent)   30.00%  
Percentage applied to 30 percent of GTN's distributions above threshold through March 31, 2020   100.00%  
Threshold of GTN's total distributable cash flows for payment to Class B units   $ 20  
Percentage applied to 30 percent of GTN's distributions above threshold after March 31, 2020   25.00%