Strategic Review of Questar Southern Trails Pipeline and Impairment of Eastern Segment (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, 2013	Dec. 31, 2012	Dec. 31, 2011
Property, Plant and Equipment [Line Items]
Net book value of Questar Southern Trails Pipeline	$ 3,603.0				$ 3,603.0	$ 3,363.2
Asset impairment charge, before income taxes	0	80.6	0	0	80.6	0	0
Questar Pipeline [Member]
Property, Plant and Equipment [Line Items]
Net book value of Questar Southern Trails Pipeline	1,147.1				1,147.1	1,210.8
Asset impairment charge, before income taxes					80.6	0	0
Questar Pipeline [Member] | Questar Southern Trails Pipeline - Western Segment [Member]
Property, Plant and Equipment [Line Items]
Net book value of Questar Southern Trails Pipeline	22.0				22.0
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