Basis of Presentation of Interim Financial Statements (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		12 Months Ended
	Mar. 31, 2014	Mar. 31, 2013	Mar. 31, 2014	Mar. 31, 2013
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Ownership percentage in unconsolidated affiliate (in hundredths)	50.00%		50.00%
Cost of Sales [Abstract]
Other cost of sales	$ 0.1	$ 0	$ 0.2	$ 0
Total cost of sales	148.2	199.7	234.4	239.3
Questar Gas [Member]
Cost of Sales [Abstract]
Gas purchases	92.7	103.3	176.0	146.0
Operator service fee	89.1	72.8	310.9	281.1
Transportation and storage	22.1	22.0	80.2	81.0
Gathering	4.9	4.6	19.1	19.0
Royalties	17.3	11.1	50.5	32.9
Storage withdrawal, net	33.0	28.8	3.4	3.3
Purchased-gas account adjustment	(5.6)	46.7	(30.3)	19.4
Other	1.1	1.3	4.8	5.0
Cost of Purchased Oil and Gas	254.6	290.6	614.6	587.7
Elimination of Questar Gas cost of natural gas sold - affiliated parties	(108.2)	(92.1)	(387.0)	(355.8)
Total Questar Gas cost of natural gas sold - unaffiliated parties	146.4	198.5	227.6	231.9
Questar Pipeline [Member]
Cost of Sales [Abstract]
Total Questar Pipeline cost of sales	$ 1.7	$ 1.2	$ 6.6	$ 7.4