Basis of Presentation of Interim Financial Statements (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Cost of Sales [Abstract]
Other cost of sales	$ 0.4	$ 0.4	$ 0.9	$ 0.7	$ 1.9	$ 1.2
Total cost of sales	(27.7)	(9.0)	130.8	122.8	183.2	185.2
Questar Gas [Member]
Cost of Sales [Abstract]
Gas purchases	2.3	2.2	49.1	43.2	88.4	83.1
Operator service fee	78.9	75.5	158.7	160.3	317.4	326.5
Transportation and storage	17.4	18.1	39.7	40.3	78.6	79.2
Gathering	5.6	5.6	11.4	11.0	22.5	22.3
Royalties	4.6	6.9	10.3	18.8	24.8	43.3
Storage (injection) withdrawal, net	(15.9)	(16.7)	20.8	14.2	3.1	1.7
Purchased-gas account adjustment	(26.5)	(8.9)	32.1	27.3	25.3	19.6
Other	1.4	1.3	2.6	2.5	5.1	4.9
Total Questar Gas cost of natural gas sold	67.8	84.0	324.7	317.6	565.2	580.6
Elimination of Questar Gas cost of natural gas sold - affiliated companies	(96.2)	(93.2)	(195.2)	(197.2)	(391.5)	(399.9)
Total Questar Gas cost of natural gas sold - unaffiliated parties	$ (28.4)	(9.2)	$ 129.5	120.4	$ 173.7	180.7
Questar Pipeline [Member]
Ownership percentage in unconsolidated affiliate (in hundredths)	50.00%		50.00%		50.00%
Cost of Sales [Abstract]
Questar Pipeline cost of sales	$ 0.3	$ (0.2)	$ 0.4	$ 1.7	$ 7.6	$ 3.3