Operations by Line of Business (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012 lines_of_business	Dec. 31, 2011	Dec. 31, 2010
Segment Reporting Information [Line Items]
Number of complementary lines of business									3
Revenues [Abstract]
From unaffiliated customers	$ 343.0	$ 141.8	$ 188.4	$ 425.7	$ 354.8	$ 146.7	$ 222.9	$ 470.0	$ 1,098.9	$ 1,194.4	$ 1,123.6
From affiliated companies									0	0	0
Total Revenues									1,098.9	1,194.4	1,123.6
Operating Expenses [Abstract]
Cost of sales									192.3	321.5	280.9
Operating and maintenance									180.8	175.9	175.8
General and administrative									120.8	117.9	108.5
Retirement incentive									4.9	0	0
Separation costs									0	0	11.5
Production and other taxes									47.9	52.5	50.6
Depreciation, depletion and amortization									181.6	159.9	153.4
Other operating expenses									0	0	0
Total Operating Expenses									728.3	827.7	780.7
Net gain (loss) from asset sales									5.1	0.2	0.4
OPERATING INCOME	108.6	63.6	72.6	130.9	106.6	63.8	73.7	122.8	375.7	366.9	343.3
Interest and other income									7.0	10.4	11.7
Income from unconsolidated affiliate									3.7	3.8	3.8
Interest expense									(57.9)	(56.8)	(57.1)
Income taxes									(116.5)	(116.4)	(109.4)
INCOME FROM CONTINUING OPERATIONS									212.0	207.9	192.3
Additional Line-of-Business Information [Abstract]
Identifiable assets of continuing operations	3,757.1				3,532.8				3,757.1	3,532.8	3,373.6
Goodwill	9.8				9.8				9.8	9.8	9.8
Investment in unconsolidated affiliate	26.5				27.3				26.5	27.3	27.9
Cash capital expenditures									370.7	367.7	320.3
Accrued capital expenditures									366.3	373.9	330.4
Interco. Trans. [Member]
Revenues [Abstract]
From unaffiliated customers									0	0	0
From affiliated companies									(351.0)	(331.3)	(315.3)
Total Revenues									(351.0)	(331.3)	(315.3)
Operating Expenses [Abstract]
Cost of sales									(347.7)	(327.3)	(313.7)
Operating and maintenance									(0.2)	(0.2)	(0.2)
General and administrative									(0.6)	(0.5)	(0.3)
Retirement incentive									0
Separation costs											0
Production and other taxes									0	0	0
Depreciation, depletion and amortization									0	0	0
Other operating expenses									(2.5)	(3.3)	(1.1)
Total Operating Expenses									(351.0)	(331.3)	(315.3)
Net gain (loss) from asset sales									0	0	0
OPERATING INCOME									0	0	0
Interest and other income									(0.7)	(0.6)	(0.8)
Income from unconsolidated affiliate									0	0	0
Interest expense									0.7	0.6	0.8
Income taxes									0	0	0
INCOME FROM CONTINUING OPERATIONS									0	0	0
Additional Line-of-Business Information [Abstract]
Identifiable assets of continuing operations	0				0				0	0	0
Goodwill	0				0				0	0	0
Investment in unconsolidated affiliate	0				0				0	0	0
Cash capital expenditures									0	0	0
Accrued capital expenditures									0	0	0
Questar Gas [Member]
Revenues [Abstract]
From unaffiliated customers									859.7	965.5	901.8
From affiliated companies									2.5	3.3	1.1
Total Revenues									862.2	968.8	902.9
Operating Expenses [Abstract]
Cost of sales									533.3	645.7	592.2
Operating and maintenance									119.0	118.5	114.4
General and administrative									51.2	51.0	49.9
Retirement incentive									2.4
Separation costs											0
Production and other taxes									16.2	15.0	14.1
Depreciation, depletion and amortization									47.2	44.5	43.7
Other operating expenses									0	0	0
Total Operating Expenses									769.3	874.7	814.3
Net gain (loss) from asset sales									0	0	0
OPERATING INCOME									92.9	94.1	88.6
Interest and other income									5.5	5.4	6.7
Income from unconsolidated affiliate									0	0	0
Interest expense									(24.1)	(25.9)	(26.2)
Income taxes									(27.2)	(27.5)	(25.2)
INCOME FROM CONTINUING OPERATIONS									47.1	46.1	43.9
Additional Line-of-Business Information [Abstract]
Identifiable assets of continuing operations	1,547.3				1,444.4				1,547.3	1,444.4	1,406.8
Goodwill	5.6				5.6				5.6	5.6	5.6
Investment in unconsolidated affiliate	0				0				0	0	0
Cash capital expenditures									162.1	121.5	108.6
Accrued capital expenditures									161.2	127.7	110.9
Wexpro [Member]
Revenues [Abstract]
From unaffiliated customers									36.1	31.5	24.6
From affiliated companies									274.1	253.6	240.2
Total Revenues									310.2	285.1	264.8
Operating Expenses [Abstract]
Cost of sales									0	0	0
Operating and maintenance									26.8	22.3	20.2
General and administrative									26.8	24.2	20.5
Retirement incentive									0.2
Separation costs											0
Production and other taxes									20.8	25.6	26.8
Depreciation, depletion and amortization									77.4	63.9	62.1
Other operating expenses									2.5	3.3	1.1
Total Operating Expenses									154.5	139.3	130.7
Net gain (loss) from asset sales									2.4	(0.1)	(0.4)
OPERATING INCOME									158.1	145.7	133.7
Interest and other income									2.8	4.2	3.2
Income from unconsolidated affiliate									0	0	0
Interest expense									0	0	(0.3)
Income taxes									(57.0)	(54.7)	(48.5)
INCOME FROM CONTINUING OPERATIONS									103.9	95.2	88.1
Additional Line-of-Business Information [Abstract]
Identifiable assets of continuing operations	836.1				768.2				836.1	768.2	667.1
Goodwill	0				0				0	0	0
Investment in unconsolidated affiliate	0				0				0	0	0
Cash capital expenditures									144.5	136.2	90.6
Accrued capital expenditures									145.5	144.3	94.1
Questar Pipeline [Member]
Revenues [Abstract]
From unaffiliated customers									203.1	197.4	197.2
From affiliated companies									74.4	74.4	74.0
Total Revenues									277.5	271.8	271.2
Operating Expenses [Abstract]
Cost of sales									6.7	3.1	2.4
Operating and maintenance									35.2	35.3	41.4
General and administrative									50.0	46.5	41.0
Retirement incentive									0.9
Separation costs											0
Production and other taxes									9.1	10.1	9.0
Depreciation, depletion and amortization									54.3	51.2	47.4
Other operating expenses									0	0	0
Total Operating Expenses									156.2	146.2	141.2
Net gain (loss) from asset sales									2.7	0.3	0.8
OPERATING INCOME									124.0	125.9	130.8
Interest and other income									0.6	0.9	0.4
Income from unconsolidated affiliate									3.7	3.8	3.8
Interest expense									(26.3)	(24.5)	(28.8)
Income taxes									(37.3)	(38.2)	(38.8)
INCOME FROM CONTINUING OPERATIONS									64.7	67.9	67.4
Additional Line-of-Business Information [Abstract]
Identifiable assets of continuing operations	1,298.9				1,286.6				1,298.9	1,286.6	1,246.9
Goodwill	4.2				4.2				4.2	4.2	4.2
Investment in unconsolidated affiliate	26.5				27.3				26.5	27.3	27.9
Cash capital expenditures									60.6	104.5	121.0
Accrued capital expenditures									58.6	93.8	125.0
Questar Corp and Other [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									(6.6)	(3.3)	(2.6)
Retirement incentive									1.4
Separation costs											11.5
Production and other taxes									1.8	1.8	0.7
Depreciation, depletion and amortization									2.7	0.3	0.2
Other operating expenses									0	0	0
Total Operating Expenses									(0.7)	(1.2)	9.8
Net gain (loss) from asset sales									0	0	0
OPERATING INCOME									0.7	1.2	(9.8)
Interest and other income									(1.2)	0.5	2.2
Income from unconsolidated affiliate									0	0	0
Interest expense									(8.2)	(7.0)	(2.6)
Income taxes									5.0	4.0	3.1
INCOME FROM CONTINUING OPERATIONS									(3.7)	(1.3)	(7.1)
Additional Line-of-Business Information [Abstract]
Identifiable assets of continuing operations	74.8				33.6				74.8	33.6	52.8
Goodwill	0				0				0	0	0
Investment in unconsolidated affiliate	0				0				0	0	0
Cash capital expenditures									3.5	5.5	0.1
Accrued capital expenditures									$ 1.0	$ 8.1	$ 0.4