Operations by Line of Business (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012 lines_of_business	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Segment Reporting Information [Line Items]
Number of complementary lines of business			3
Revenues from unaffiliated customers	$ 141.8	$ 146.7	$ 755.9	$ 839.6	$ 1,110.7	$ 1,202.3
Revenues from affiliated companies	88.2	83.3	262.1	244.4	349.0	324.5
Operating income (loss)	63.6	63.8	267.1	260.3	373.7	368.1
Net income (loss)	33.8	36.1	148.2	146.3	209.8	210.0
Questar Gas [Member]
Segment Reporting Information [Line Items]
Revenues from unaffiliated customers	82.5	89.8	578.7	667.7	876.5	970.4
Revenues from affiliated companies	0.5	0.4	2.3	2.2	3.4	2.7
Operating income (loss)	(10.7)	(7.9)	52.6	56.6	90.1	95.8
Net income (loss)	(9.3)	(7.9)	23.3	25.9	43.5	48.0
Wexpro [Member]
Segment Reporting Information [Line Items]
Revenues from unaffiliated customers	9.8	6.9	27.1	23.1	35.5	31.1
Revenues from affiliated companies	69.3	64.4	204.6	186.4	271.8	247.5
Operating income (loss)	41.0	37.7	117.9	108.3	155.3	143.1
Net income (loss)	26.4	25.6	76.5	71.6	100.1	94.3
Questar Pipeline [Member]
Segment Reporting Information [Line Items]
Revenues from unaffiliated customers	49.5	50.0	150.1	148.8	198.7	200.8
Revenues from affiliated companies	18.4	18.5	55.2	55.8	73.8	74.3
Operating income (loss)	32.6	33.9	95.2	94.9	126.2	129.0
Net income (loss)	17.1	18.8	49.8	50.7	67.0	68.7
Questar Corp [Member]
Segment Reporting Information [Line Items]
Operating income (loss)	0.7	0.1	1.4	0.5	2.1	0.2
Net income (loss)	$ (0.4)	$ (0.4)	$ (1.4)	$ (1.9)	$ (0.8)	$ (1.0)