DERIVATIVES AND HEDGE ACCOUNTING ACTIVITIES - Schedule of Gains and Losses on Derivatives (Details) - Derivatives not designated as hedging instruments - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016
Derivative Instruments Gain Loss [Line Items]
Amount of Gain (Loss) Recognized in Income on Derivatives	[1]	$ (0.1)	$ (0.2)
Commodity
Derivative Instruments Gain Loss [Line Items]
Amount of Gain (Loss) Recognized in Income on Derivatives	[1],[2]	$ (0.1)	$ (0.2)

[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 purchased gas.