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RELATED-PARTY TRANSACTIONS (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Related Party Transaction [Line Items]      
Affiliated current borrowings $ 75.0 $ 48.0  
Dominion | IRCA | Revolving Credit Facility      
Related Party Transaction [Line Items]      
Weighted-average interest rate percentage 1.53% 1.04%  
Interest charges $ 0.2 $ 1.3  
Transportation and storage services from affiliates      
Related Party Transaction [Line Items]      
Related party expenses in cost of sales [1] 73.7 72.9 $ 73.0
Services provided by/to related parties      
Related Party Transaction [Line Items]      
Related party expenses 50.2 65.0 55.7
Related party revenues 6.0 3.2 6.7
Merger and restructuring costs | Dominion Energy Questar Corporation      
Related Party Transaction [Line Items]      
Transaction costs $ 9.8 $ 13.8 $ 0.0
[1] The costs of these services were included in purchased gas in Questar Gas' Statements of Income.