Operations by Line of Business (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Segment Reporting Information [Line Items]
Number of complementary lines of business			3
Revenues from unaffiliated customers	$ 188.4	$ 222.9	$ 614.1	$ 692.9	$ 1,115.6	$ 1,200.8
Revenues from affiliated companies	88.9	80.8	173.9	161.1	344.1	318.0
Operating income	72.6	73.7	203.5	196.5	373.9	358.7
Net income (loss)	39.2	40.3	114.4	110.2	212.1	201.6
Questar Gas [Member]
Segment Reporting Information [Line Items]
Revenues from unaffiliated customers	130.2	164.0	496.2	577.9	883.8	970.4
Revenues from affiliated companies	1.0	1.0	1.8	1.8	3.3	2.5
Operating income	1.5	5.7	63.3	64.5	92.9	93.4
Net income (loss)	(2.3)	0.4	32.6	33.8	44.9	46.8
Wexpro [Member]
Segment Reporting Information [Line Items]
Revenues from unaffiliated customers	8.1	8.4	17.3	16.2	32.6	30.3
Revenues from affiliated companies	69.6	61.1	135.3	122.0	266.9	241.5
Operating income	39.7	36.1	76.9	70.6	152.0	138.2
Net income (loss)	25.8	23.7	50.1	46.0	99.3	90.9
Questar Pipeline [Member]
Segment Reporting Information [Line Items]
Revenues from unaffiliated customers	50.1	50.5	100.6	98.8	199.2	200.1
Revenues from affiliated companies	18.3	18.7	36.8	37.3	73.9	74.0
Operating income	30.8	31.7	62.6	61.0	127.5	127.0
Net income (loss)	16.1	16.6	32.7	31.9	68.7	66.2
Questar Corp [Member]
Segment Reporting Information [Line Items]
Operating income	0.6	0.2	0.7	0.4	1.5	0.1
Net income (loss)	$ (0.4)	$ (0.4)	$ (1.0)	$ (1.5)	$ (0.8)	$ (2.3)