Asset Impairments (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended				12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Property, Plant and Equipment [Line Items]
Asset impairment charge, before income taxes	$ 0	$ 80.6	$ 0	$ 0	$ 2.0	$ 80.6	$ 0
Wexpro [Member]
Property, Plant and Equipment [Line Items]
Asset impairment charge, before income taxes					2.0
Wexpro [Member] | Wexpro Brady Field [Member]
Property, Plant and Equipment [Line Items]
Asset impairment charge, before income taxes					2.0
Questar Pipeline [Member]
Property, Plant and Equipment [Line Items]
Asset impairment charge, before income taxes					0	80.6	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