Basis of Presentation of Interim Financial Statements (Details) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2015 USD ($)	Sep. 30, 2014 USD ($)	Sep. 30, 2015 USD ($) customers	Sep. 30, 2014 USD ($)	Sep. 30, 2015 USD ($)	Sep. 30, 2014 USD ($)	Mar. 31, 2015 USD ($)
Cost of Sales [Abstract]
Other cost of sales	$ 0.6	$ 0.2	$ 1.3	$ 0.6	$ 1.6	$ 0.7
Total cost of sales	(52.6)	(42.3)	$ 70.2	81.6	174.9	210.1
Questar Gas [Member]
Payments for Questar Gas Acquisition							$ 11.4
Additional Customers from Questar Gas Acquisition | customers			6,500
Cost of Sales [Abstract]
Gas purchases	0.3	0.1	$ 43.5	96.7	83.3	169.6
Operator service fee	80.7	82.3	241.0	265.8	324.9	345.2
Transportation and storage	18.1	18.1	58.4	58.8	79.2	80.3
Gathering	5.7	4.5	16.7	14.2	23.5	19.4
Royalties	7.6	14.9	26.4	50.5	36.0	62.4
Storage (injection),net	(42.8)	(39.0)	(28.6)	(27.6)	(2.1)	(10.3)
Purchased-gas account adjustment	(26.4)	(25.3)	0.9	(63.4)	18.5	(46.5)
Other	1.2	1.2	3.7	3.6	4.9	4.9
Total Questar Gas cost of natural gas sold	44.4	56.8	362.0	398.6	568.2	625.0
Elimination of Questar Gas cost of natural gas sold - affiliated companies	(98.2)	(99.9)	(295.4)	(320.6)	(398.2)	(422.0)
Total Questar Gas cost of natural gas sold - unaffiliated parties	$ (53.8)	(43.1)	$ 66.6	78.0	$ 170.0	203.0
Questar Pipeline [Member]
Ownership percentage in unconsolidated affiliate (in hundredths)	50.00%		50.00%		50.00%
Cost of Sales [Abstract]
Total Questar Pipeline cost of sales	$ 0.6	$ 0.6	$ 2.3	$ 3.0	$ 3.3	$ 6.4