Operations by Line of Business (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		12 Months Ended
	Mar. 31, 2012	Mar. 31, 2011	Mar. 31, 2012	Mar. 31, 2011
Segment Reporting Information [Line Items]
Number of complementary lines of business	3
Revenues from unaffiliated customers	$ 425.7	$ 470.0	$ 1,150.1	$ 1,179.0
Revenues from affiliated companies	85.0	80.3	336.0	316.0
Operating income (loss)	130.9	122.8	375.0	341.3
Income (loss) from continuing operations	75.2	69.9	213.2	190.0
Questar Gas [Member]
Segment Reporting Information [Line Items]
Revenues from unaffiliated customers	366.0	413.9	917.6	955.0
Revenues from affiliated companies	0.8	0.8	3.3	1.6
Operating income (loss)	61.8	58.8	97.1	88.8
Income (loss) from continuing operations	34.9	33.4	47.6	44.2
Wexpro [Member]
Segment Reporting Information [Line Items]
Revenues from unaffiliated customers	9.2	7.8	32.9	26.2
Revenues from affiliated companies	65.7	60.9	258.4	240.6
Operating income (loss)	37.2	34.5	148.4	135.4
Income (loss) from continuing operations	24.3	22.3	97.2	89.2
Questar Pipeline [Member]
Segment Reporting Information [Line Items]
Revenues from unaffiliated customers	50.5	48.3	199.6	197.8
Revenues from affiliated companies	18.5	18.6	74.3	73.8
Operating income (loss)	31.8	29.3	128.4	126.6
Income (loss) from continuing operations	16.6	15.3	69.2	65.5
Questar Corp [Member]
Segment Reporting Information [Line Items]
Operating income (loss)	0.1	0.2	1.1	(9.5)
Income (loss) from continuing operations	$ (0.6)	$ (1.1)	$ (0.8)	$ (8.9)