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REGULATORY (Details) - USD ($)
$ in Millions
6 Months Ended 12 Months Ended
Jun. 30, 2018
Dec. 31, 2017
FERC | Maximum    
REGULATORY    
Estimated ROE   12.00%
Tuscarora Gas Transmission Company | Maximum    
REGULATORY    
Percentage of fair value exceeding its carrying value   10.00%
Tuscarora Gas Transmission Company | FERC    
REGULATORY    
Goodwill impairment charge $ 82 $ 82
Great Lakes | Maximum    
REGULATORY    
Percentage of fair value exceeding its carrying value   10.00%
Great Lakes | FERC    
REGULATORY    
Goodwill impairment charge $ 260 $ 260