Basis of Presentation of Interim Financial Statements (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Ownership percentage in unconsolidated affiliate (in hundredths)	50.00%		50.00%		50.00%
Cost of Sales [Abstract]
Other cost of sales	$ 0.2	$ 0	$ 0.6	$ 0	$ 0.7	$ 0
Total cost of sales	(42.3)	(35.0)	81.6	157.4	210.1	250.6
Questar Gas [Member]
Cost of Sales [Abstract]
Gas purchases	0.1	0.1	96.7	113.7	169.6	153.6
Operator service fee	82.3	68.2	265.8	215.2	345.2	284.7
Transportation and storage	18.1	18.5	58.8	58.6	80.3	79.7
Gathering	4.5	4.5	14.2	13.6	19.4	18.1
Royalties	14.9	11.0	50.5	32.4	62.4	41.0
Storage (injection) withdrawal, net	(39.0)	(37.3)	(27.6)	(18.1)	(10.3)	1.9
Purchased-gas account adjustment	(25.3)	(15.9)	(63.4)	5.1	(46.5)	17.2
Other	1.2	1.0	3.6	3.7	4.9	4.9
Total Questar Gas cost of natural gas sold	56.8	50.1	398.6	424.2	625.0	601.1
Elimination of Questar Gas cost of natural gas sold - affiliated companies	(99.9)	(85.6)	(320.6)	(269.5)	(422.0)	(358.0)
Total Questar Gas cost of natural gas sold - unaffiliated parties	(43.1)	(35.5)	78.0	154.7	203.0	243.1
Questar Pipeline [Member]
Cost of Sales [Abstract]
Total Questar Pipeline cost of sales	$ 0.6	$ 0.5	$ 3.0	$ 2.7	$ 6.4	$ 7.5