Strategic Review of Questar Southern Trails Pipeline and Impairment of Eastern Segment (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012
Property, Plant and Equipment [Line Items]
Net book value of Questar Southern Trails Pipeline	$ 3,495.0	$ 3,261.2	$ 3,495.0	$ 3,261.2	$ 3,495.0	$ 3,261.2	$ 3,317.0
Pre-tax asset impairment charge	80.6	0	80.6	0	80.6	0
Questar Pipeline [Member]
Property, Plant and Equipment [Line Items]
Net book value of Questar Southern Trails Pipeline	1,148.3	1,203.2	1,148.3	1,203.2	1,148.3	1,203.2	1,210.8
Pre-tax asset impairment charge	80.6	0	80.6	0	80.6	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	21.0		21.0		21.0
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