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INCOME TAXES (Details) - USD ($)
$ in Millions
3 Months Ended 12 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Dec. 31, 2017
State income taxes      
Total state income taxes $ 1 $ 1 [1]  
Portland Natural Gas Transmission System      
Income Taxes      
Effective income tax rate (as a percent) 3.80%   3.80%
State income taxes      
Current $ 1 1  
Total state income taxes $ 1 $ 1  
[1] Recast to consolidate PNGTS (Refer to Note 2).