Operations by Line of Business (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011 lines_of_business	Dec. 31, 2010	Dec. 31, 2009
Segment Reporting Information [Line Items]
Number of complementary lines of business	3
Revenues [Abstract]
From unaffiliated customers	$ 1,194.4	$ 1,123.6	$ 1,109.9
From affiliated companies	0	0	0
Total Revenues	1,194.4	1,123.6	1,109.9
Operating expenses [Abstract]
Cost of sales	321.5	280.9	331.4
Operating and maintenance	175.9	175.8	167.6
General and administrative	117.9	108.5	93.4
Separation costs	0	11.5	0
Production and other taxes	52.5	50.6	42.4
Depreciation, depletion and amortization	159.9	153.4	147.1
Other operating expenses	0	0	0
Total Operating Expenses	827.7	780.7	781.9
Net gain (loss) from asset sales	0.2	0.4	0.2
Operating Income (loss)	366.9	343.3	328.2
Interest and other income	10.4	11.7	12.5
Income from unconsolidated affiliate	3.8	3.8	3.8
Interest expense	(56.8)	(57.1)	(59.6)
Income taxes	(116.4)	(109.4)	(104.4)
Income from Continuing Operations	207.9	192.3	180.5
Segment Reporting Information, Additional Information [Abstract]
Identifiable assets of continuing operations	3,532.8	3,373.6	3,189.7
Goodwill	9.8	9.8	9.8
Investment in unconsolidated affiliate	27.3	27.9	28.1
Cash capital expenditures	367.7	320.3	299.8
Accrued capital expenditures	373.9	330.4	292.4
Interco. Trans. [Member]
Revenues [Abstract]
From unaffiliated customers	0	0	0
From affiliated companies	(331.3)	(315.3)	(298.3)
Total Revenues	(331.3)	(315.3)	(298.3)
Operating expenses [Abstract]
Cost of sales	(327.3)	(313.7)	(296.8)
Operating and maintenance	(0.2)	(0.2)	(0.1)
General and administrative	(0.5)	(0.3)	(0.4)
Separation costs		0
Production and other taxes	0	0	0
Depreciation, depletion and amortization	0	0	0
Other operating expenses	(3.3)	(1.1)	(1.0)
Total Operating Expenses	(331.3)	(315.3)	(298.3)
Net gain (loss) from asset sales	0	0	0
Operating Income (loss)	0	0	0
Interest and other income	(0.6)	(0.8)	(0.9)
Income from unconsolidated affiliate	0	0	0
Interest expense	0.6	0.8	0.9
Income taxes	0	0	0
Income from Continuing Operations	0	0	0
Segment Reporting Information, Additional Information [Abstract]
Identifiable assets of continuing operations	0	0	0
Goodwill	0	0	0
Investment in unconsolidated affiliate	0	0	0
Cash capital expenditures	0	0	0
Accrued capital expenditures	0	0	0
Wexpro [Member]
Revenues [Abstract]
From unaffiliated customers	31.5	24.6	17.8
From affiliated companies	253.6	240.2	225.1
Total Revenues	285.1	264.8	242.9
Operating expenses [Abstract]
Cost of sales	0	0	0
Operating and maintenance	22.3	20.2	21.2
General and administrative	24.2	20.5	17.0
Separation costs		0
Production and other taxes	25.6	26.8	20.0
Depreciation, depletion and amortization	63.9	62.1	58.8
Other operating expenses	3.3	1.1	1.0
Total Operating Expenses	139.3	130.7	118.0
Net gain (loss) from asset sales	(0.1)	(0.4)	(0.3)
Operating Income (loss)	145.7	133.7	124.6
Interest and other income	4.2	3.2	3.2
Income from unconsolidated affiliate	0	0	0
Interest expense	0	(0.3)	(0.9)
Income taxes	(54.7)	(48.5)	(46.2)
Income from Continuing Operations	95.2	88.1	80.7
Segment Reporting Information, Additional Information [Abstract]
Identifiable assets of continuing operations	768.2	667.1	621.5
Goodwill	0	0	0
Investment in unconsolidated affiliate	0	0	0
Cash capital expenditures	136.2	90.6	116.2
Accrued capital expenditures	144.3	94.1	110.1
Questar Pipeline [Member]
Revenues [Abstract]
From unaffiliated customers	197.4	197.2	173.2
From affiliated companies	74.4	74.0	72.2
Total Revenues	271.8	271.2	245.4
Operating expenses [Abstract]
Cost of sales	3.1	2.4	1.6
Operating and maintenance	35.3	41.4	40.1
General and administrative	46.5	41.0	36.1
Separation costs		0
Production and other taxes	10.1	9.0	8.6
Depreciation, depletion and amortization	51.2	47.4	44.3
Other operating expenses	0	0	0
Total Operating Expenses	146.2	141.2	130.7
Net gain (loss) from asset sales	0.3	0.8	0.5
Operating Income (loss)	125.9	130.8	115.2
Interest and other income	0.9	0.4	2.5
Income from unconsolidated affiliate	3.8	3.8	3.8
Interest expense	(24.5)	(28.8)	(29.5)
Income taxes	(38.2)	(38.8)	(33.8)
Income from Continuing Operations	67.9	67.4	58.2
Segment Reporting Information, Additional Information [Abstract]
Identifiable assets of continuing operations	1,286.6	1,246.9	1,165.4
Goodwill	4.2	4.2	4.2
Investment in unconsolidated affiliate	27.3	27.9	28.1
Cash capital expenditures	104.5	121.0	100.8
Accrued capital expenditures	93.8	125.0	94.5
Questar Gas [Member]
Revenues [Abstract]
From unaffiliated customers	965.5	901.8	918.9
From affiliated companies	3.3	1.1	1.0
Total Revenues	968.8	902.9	919.9
Operating expenses [Abstract]
Cost of sales	645.7	592.2	626.6
Operating and maintenance	118.5	114.4	106.4
General and administrative	51.0	49.9	42.9
Separation costs		0
Production and other taxes	15.0	14.1	13.3
Depreciation, depletion and amortization	44.5	43.7	43.8
Other operating expenses	0	0	0
Total Operating Expenses	874.7	814.3	833.0
Net gain (loss) from asset sales	0	0	0
Operating Income (loss)	94.1	88.6	86.9
Interest and other income	5.4	6.7	7.6
Income from unconsolidated affiliate	0	0	0
Interest expense	(25.9)	(26.2)	(28.5)
Income taxes	(27.5)	(25.2)	(24.4)
Income from Continuing Operations	46.1	43.9	41.6
Segment Reporting Information, Additional Information [Abstract]
Identifiable assets of continuing operations	1,444.4	1,406.8	1,335.2
Goodwill	5.6	5.6	5.6
Investment in unconsolidated affiliate	0	0	0
Cash capital expenditures	121.5	108.6	82.6
Accrued capital expenditures	127.7	110.9	87.6
Questar Corp [Member]
Revenues [Abstract]
From unaffiliated customers	0	0	0
From affiliated companies	0	0	0
Total Revenues	0	0	0
Operating expenses [Abstract]
Cost of sales	0	0	0
Operating and maintenance	0	0	0
General and administrative	(3.3)	(2.6)	(2.2)
Separation costs		11.5
Production and other taxes	1.8	0.7	0.5
Depreciation, depletion and amortization	0.3	0.2	0.2
Other operating expenses	0	0	0
Total Operating Expenses	(1.2)	9.8	(1.5)
Net gain (loss) from asset sales	0	0	0
Operating Income (loss)	1.2	(9.8)	1.5
Interest and other income	0.5	2.2	0.1
Income from unconsolidated affiliate	0	0	0
Interest expense	(7.0)	(2.6)	(1.6)
Income taxes	4.0	3.1	0
Income from Continuing Operations	(1.3)	(7.1)	0
Segment Reporting Information, Additional Information [Abstract]
Identifiable assets of continuing operations	33.6	52.8	67.6
Goodwill	0	0	0
Investment in unconsolidated affiliate	0	0	0
Cash capital expenditures	5.5	0.1	0.2
Accrued capital expenditures	$ 8.1	$ 0.4	$ 0.2