Asset Impairments (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		12 Months Ended		3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Mar. 31, 2015	Mar. 31, 2014	Jun. 30, 2014	Sep. 30, 2013
Property, Plant and Equipment [Line Items]
Pre-tax asset impairment charge	$ 0	$ 0	$ 2.0	$ 80.6
Wexpro [Member] | Wexpro Brady Field [Member]
Property, Plant and Equipment [Line Items]
Pre-tax asset impairment charge					2.0
Questar Pipeline [Member]
Property, Plant and Equipment [Line Items]
Pre-tax asset impairment charge	0	0	0	80.6
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