Asset Impairments (Details) - USD ($) $ in Millions	3 Months Ended			6 Months Ended		12 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Sep. 30, 2013	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Property, Plant and Equipment [Line Items]
Pre-tax asset impairment charge	$ 0.1	$ 2.0		$ 0.1	$ 2.0	$ 0.1	$ 82.6
Net (gain) from asset sales	1.4	0.1		1.4	0.1	2.5	(0.2)
Wexpro [Member] | Wexpro Brady Field [Member]
Property, Plant and Equipment [Line Items]
Pre-tax asset impairment charge		2.0
Net (gain) from asset sales	1.4
Questar Pipeline [Member]
Property, Plant and Equipment [Line Items]
Pre-tax asset impairment charge	0.0	0.0		0.0	0.0	0.0	80.6
Net (gain) from asset sales	$ 0.0	$ 0.0		$ 0.0	$ 0.0	$ (0.5)	$ 0.0
Questar Pipeline [Member] | Questar Southern Trails Pipeline - Eastern Segment [Member]
Property, Plant and Equipment [Line Items]
Pre-tax asset impairment charge			$ 80.6
After-tax asset impairment charge			$ 52.4