Operations by Line of Business (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010
Operations by Line of Business [Abstract]
Number of complementary lines of business (in lines)			3
Segment Reporting Information [Line Items]
Revenues from unaffiliated customers	$ 222.9	$ 201.1	$ 692.9	$ 615.7	$ 1,200.8	$ 1,090.6
Revenues from affiliated companies	80.8	78.8	161.1	158.4	318.0	307.3
Operating income (loss)	73.7	56.3	196.5	181.1	358.7	332.4
Income (loss) from continuing operations	40.3	28.7	110.2	100.9	201.6	183.1
Wexpro [Member]
Segment Reporting Information [Line Items]
Revenues from unaffiliated customers	8.4	4.3	16.2	10.5	30.3	22.5
Revenues from affiliated companies	61.1	60.2	122.0	120.7	241.5	233.0
Operating income (loss)	36.1	33.3	70.6	66.1	138.2	131.9
Income (loss) from continuing operations	23.7	22.0	46.0	43.2	90.9	85.3
Questar Pipeline [Member]
Segment Reporting Information [Line Items]
Revenues from unaffiliated customers	50.5	48.2	98.8	95.9	200.1	184.1
Revenues from affiliated companies	18.7	18.5	37.3	37.3	74.0	73.4
Operating income (loss)	31.7	31.3	61.0	64.8	127.0	120.8
Income (loss) from continuing operations	16.6	15.9	31.9	33.1	66.2	61.6
Questar Gas [Member]
Segment Reporting Information [Line Items]
Revenues from unaffiliated customers	164.0	148.6	577.9	509.3	970.4	884.0
Revenues from affiliated companies	1.0	0.1	1.8	0.4	2.5	0.9
Operating income (loss)	5.7	1.1	64.5	59.7	93.4	87.8
Income (loss) from continuing operations	0.4	(2.2)	33.8	30.9	46.8	42.7
Corporate [Member]
Segment Reporting Information [Line Items]
Operating income (loss)	0.2	(9.4)	0.4	(9.5)	0.1	(8.1)
Income (loss) from continuing operations	$ (0.4)	$ (7.0)	$ (1.5)	$ (6.3)	$ (2.3)	$ (6.5)