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NET INCOME PER COMMON UNIT- Terms of Class B Unit Distributions and Determination of Net Income (Loss) per Common Unit (Details) - USD ($)
$ / shares in Units, shares in Millions, $ in Millions
3 Months Ended 12 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Dec. 31, 2017
Net income (loss) per common unit      
Net income attributable to controlling interests $ 96 $ 77 [1]  
Net income attributable to PNGTS' former parent [1]   (2)  
Net income allocable to General Partner and Limited Partners 96 75  
Net income attributable to the General Partner (2) (1)  
Incentive distributions attributable to the General Partner   (2)  
Common Units      
Net income (loss) per common unit      
Net income attributable to common units $ 94 $ 72 [1]  
Weighted average common units outstanding - basic (in units) 71.2 68.3 [1]  
Weighted average common units outstanding - diluted (in units) 71.2 68.3  
Net income (loss) per common unit - basic (in dollars per unit) [2] $ 1.32 $ 1.05 [1]  
Net income (loss) per common unit - diluted (in dollars per unit) $ 1.32 $ 1.05  
GTN | Class B Units | TransCanada | Distributions      
Distributions      
Percentage applied to GTN's distributable cash flow 30.00%    
Net income attributable to controlling interests $ 0 $ 0  
Threshold of GTN's distributions for payment to Class B units $ 20   $ 20
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%    
[1] Recast to consolidate PNGTS (Refer to Note 2).
[2] Net income per common unit prior to recast (Refer to Note 2).