Basis of Presentation of Interim Financial Statements (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 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	$ 0.3	$ 0	$ 0.4	$ 0	$ 0.5	$ 0
Total cost of sales	(24.3)	(7.3)	123.9	192.4	217.4	243.5
Questar Gas [Member]
Cost of Sales [Abstract]
Gas purchases	3.9	10.3	96.6	113.6	169.6	153.5
Operator service fee	94.4	74.2	183.5	147.0	331.1	285.8
Transportation and storage	18.6	18.1	40.7	40.1	80.7	80.2
Gathering	4.8	4.5	9.7	9.1	19.4	18.4
Royalties	18.3	10.3	35.6	21.4	58.5	37.0
Storage (injection) withdrawal, net	(21.6)	(9.6)	11.4	19.2	(8.6)	10.2
Purchased-gas account adjustment	(32.5)	(25.7)	(38.1)	21.0	(37.1)	5.5
Other	1.3	1.4	2.4	2.7	4.7	5.1
Cost of Purchased Oil and Gas	87.2	83.5	341.8	374.1	618.3	595.7
Elimination of Questar Gas cost of natural gas sold - affiliated parties	(112.5)	(91.8)	(220.7)	(183.9)	(407.7)	(359.9)
Total Questar Gas cost of natural gas sold - unaffiliated parties	(25.3)	(8.3)	121.1	190.2	210.6	235.8
Questar Pipeline [Member]
Cost of Sales [Abstract]
Total Questar Pipeline cost of sales	$ 0.7	$ 1.0	$ 2.4	$ 2.2	$ 6.3	$ 7.7