Derivatives and Hedge Accounting Activities - Schedule of Gains and Losses on Derivatives (Details) - Derivatives not designated as hedging instruments - USD ($) $ in Millions		9 Months Ended
		Sep. 30, 2018	Sep. 30, 2017
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Gain (Loss) Recognized in Income on Derivatives	[1]	$ 0.1	$ (0.5)
Commodity
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Gain (Loss) Recognized in Income on Derivatives	[1],[2]	$ 0.1	$ (0.5)

[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 purchased gas.