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Derivatives and Hedge Accounting Activities - Schedule of Gains and Losses on Derivatives (Details) - USD ($)
3 Months Ended 9 Months Ended
Sep. 30, 2017
Sep. 30, 2016
Sep. 30, 2017
Derivative Instruments, Gain (Loss) [Line Items]      
Amount of Gain (Loss) Recognized in Income on Derivatives $ 0 $ 0  
Derivatives not designated as hedging instruments      
Derivative Instruments, Gain (Loss) [Line Items]      
Amount of Gain (Loss) Recognized in Income on Derivatives [1]     $ (500,000)
Derivatives not designated as hedging instruments | Commodity      
Derivative Instruments, Gain (Loss) [Line Items]      
Amount of Gain (Loss) Recognized in Income on Derivatives [1],[2]     $ (500,000)
[1] Includes derivative activity amortized out of regulatory assets/liabilities. Amounts deferred into regulatory assets/liabilities have no associated effect in Questar Gas' Statements of Income.
[2] Amounts recorded in Questar Gas' Statements of Income are classified in cost of sales.