Basis of Presentation of Interim Financial Statements (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		12 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Mar. 31, 2013	Mar. 31, 2012
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Ownership percentage in unconsolidated affiliate (in hundredths)	50.00%		50.00%
Cost of Sales [Abstract]
Total cost of sales	$ 199.7	$ 152.7	$ 239.3	$ 264.3
Questar Gas [Member]
Cost of Sales [Abstract]
Gas purchases	103.3	61.5	146.0	164.2
Operator service fee	72.8	65.7	281.1	258.3
Transportation and storage	22.0	20.6	81.0	78.0
Gathering	4.6	6.1	19.0	25.2
Royalties	11.1	10.2	32.9	40.7
Storage (injection) withdrawal, net	28.8	27.4	3.3	(2.3)
Purchased-gas account adjustment	46.7	43.4	19.4	24.0
Other	1.3	1.3	5.0	5.1
Total Questar Gas cost of natural gas sold	290.6	236.2	587.7	593.2
Elimination of Questar Gas cost of natural gas sold - affiliated parties	(92.1)	(84.0)	(355.8)	(331.9)
Total Questar Gas cost of natural gas sold - unaffiliated parties	198.5	152.2	231.9	261.3
Questar Pipeline [Member]
Cost of Sales [Abstract]
Total Questar Pipeline cost of sales	$ 1.2	$ 0.5	$ 7.4	$ 3.0