Basis of Presentation of Interim Financial Statements (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2016	Mar. 31, 2015	Mar. 31, 2016	Mar. 31, 2015
Cost of Sales [Abstract]
Other cost of sales	$ 0.5	$ 0.3	$ 1.9	$ 1.1
Total cost of sales	158.5	131.8	201.9	169.9
Questar Gas [Member]
Cost of Sales [Abstract]
Gas purchases	46.8	41.0	88.3	84.8
Operator service fee	79.8	84.8	314.0	345.4
Transportation and storage	22.3	22.2	79.3	79.7
Gathering	5.8	5.4	22.5	21.5
Royalties	5.7	11.9	27.1	54.7
Storage (injection) withdrawal, net	36.7	30.9	2.3	(3.2)
Purchased-gas account adjustment	58.6	36.2	42.9	(4.0)
Other	1.2	1.2	5.0	4.9
Total Questar Gas cost of natural gas sold	256.9	233.6	581.4	583.8
Elimination of Questar Gas cost of natural gas sold - affiliated companies	(99.0)	(104.0)	(388.5)	(419.2)
Total Questar Gas cost of natural gas sold - unaffiliated parties	$ 157.9	129.6	$ 192.9	164.6
Questar Pipeline [Member]
Ownership percentage in unconsolidated affiliate (in hundredths)	50.00%		50.00%
Cost of Sales [Abstract]
Questar Pipeline cost of sales	$ 0.1	$ 1.9	$ 7.1	$ 4.2