Derivatives and Hedge Accounting Activities - Schedule of Gains and Losses on Derivatives (Details) - Derivatives not designated as hedging instruments $ in Millions	6 Months Ended
	Jun. 30, 2017 USD ($)	[1]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Gain (Loss) Recognized in Income on Derivatives	$ (0.5)
Commodity
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Gain (Loss) Recognized in Income on Derivatives	$ (0.5)	[2]

[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.