Asset Impairments (Details) - USD ($) $ in Millions	3 Months Ended						12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Jun. 30, 2014	Sep. 30, 2013	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Property, Plant and Equipment [Line Items]
Asset impairment charge, before income taxes							$ 12.5	$ 2.0	$ 80.6
Net gain (loss) from asset sales							1.8	1.2	(0.2)
Wexpro [Member]
Property, Plant and Equipment [Line Items]
Asset impairment charge, before income taxes	$ 12.1	$ 0.0	$ 0.0	$ 0.0			12.1
Wexpro [Member] | Wexpro Brady Field [Member]
Property, Plant and Equipment [Line Items]
Asset impairment charge, before income taxes					$ 2.0
Net gain (loss) from asset sales			$ 1.4
Questar Pipeline [Member]
Property, Plant and Equipment [Line Items]
Asset impairment charge, before income taxes						$ 80.6	0.0	0.0	80.6
Net gain (loss) from asset sales							$ 0.2	$ (0.5)	$ 0.0
Questar Pipeline [Member] | Questar Southern Trails Pipeline - Eastern Segment [Member]
Property, Plant and Equipment [Line Items]
Asset impairment charge, before income taxes						80.6
After-tax asset impairment charge						$ 52.4