Operations by Line of Business (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013 lines_of_business	Dec. 31, 2012	Dec. 31, 2011
Segment Reporting Information [Line Items]
Number of complementary lines of business									3
Revenues [Abstract]
Total Revenues	$ 397.9	$ 149.6	$ 195.6	$ 476.9	$ 343.0	$ 141.8	$ 188.4	$ 425.7	$ 1,220.0	$ 1,098.9	$ 1,194.4
Operating Expenses [Abstract]
Cost of sales									285.9	192.3	321.5
Operating and maintenance									174.3	180.8	175.9
General and administrative									121.0	120.8	117.9
Retirement incentive									0	4.9	0
Production and other taxes									57.4	47.9	52.5
Depreciation, depletion and amortization									194.8	181.6	159.9
Asset impairment	0	80.6	0	0					80.6	0	0
Other operating expenses									0	0	0
Total Operating Expenses									914.0	728.3	827.7
Net gain (loss) from asset sales									(0.2)	5.1	0.2
OPERATING INCOME	122.6	(19.4)	71.2	131.4	108.6	63.6	72.6	130.9	305.8	375.7	366.9
Interest and other income									9.9	7.0	10.4
Income from unconsolidated affiliate									3.7	3.7	3.8
Interest expense									(56.9)	(57.9)	(56.8)
Income taxes									(101.3)	(116.5)	(116.4)
NET INCOME	68.1	(19.2)	39.4	72.9	63.8	33.8	39.2	75.2	161.2	212.0	207.9
Additional Line-of-Business Information [Abstract]
Identifiable assets	4,054.3				3,803.3				4,054.3	3,803.3	3,572.2
Goodwill	9.8				9.8				9.8	9.8	9.8
Investment in unconsolidated affiliate	25.6				26.5				25.6	26.5	27.3
Cash capital expenditures including acquisitions									503.7	370.7	367.7
Accrued capital expenditures including acquisitions									505.6	366.3	373.9
From unaffiliated customers [Member]
Revenues [Abstract]
Total Revenues									1,220.0	1,098.9	1,194.4
From affiliated companies [Member]
Revenues [Abstract]
Total Revenues									0	0	0
Operating Segments [Member] | Questar Gas [Member]
Revenues [Abstract]
Total Revenues									985.8	862.2	968.8
Operating Expenses [Abstract]
Cost of sales									650.6	533.3	645.7
Operating and maintenance									113.1	119.0	118.5
General and administrative									52.5	51.2	51.0
Retirement incentive										2.4
Production and other taxes									18.0	16.2	15.0
Depreciation, depletion and amortization									49.7	47.2	44.5
Asset impairment									0
Other operating expenses									0	0	0
Total Operating Expenses									883.9	769.3	874.7
Net gain (loss) from asset sales									0	0	0
OPERATING INCOME									101.9	92.9	94.1
Interest and other income									5.1	5.5	5.4
Income from unconsolidated affiliate									0	0	0
Interest expense									(22.3)	(24.1)	(25.9)
Income taxes									(31.9)	(27.2)	(27.5)
NET INCOME									52.8	47.1	46.1
Additional Line-of-Business Information [Abstract]
Identifiable assets	1,762.0				1,593.5				1,762.0	1,593.5	1,483.8
Goodwill	5.6				5.6				5.6	5.6	5.6
Investment in unconsolidated affiliate	0				0				0	0	0
Cash capital expenditures including acquisitions									166.2	162.1	121.5
Accrued capital expenditures including acquisitions									177.3	161.2	127.7
Operating Segments [Member] | Questar Gas [Member] | From unaffiliated customers [Member]
Revenues [Abstract]
Total Revenues									985.2	859.7	965.5
Operating Segments [Member] | Questar Gas [Member] | From affiliated companies [Member]
Revenues [Abstract]
Total Revenues									0.6	2.5	3.3
Operating Segments [Member] | Wexpro [Member]
Revenues [Abstract]
Total Revenues									339.9	310.2	285.1
Operating Expenses [Abstract]
Cost of sales									0	0	0
Operating and maintenance									28.6	26.8	22.3
General and administrative									28.7	26.8	24.2
Retirement incentive										0.2
Production and other taxes									28.3	20.8	25.6
Depreciation, depletion and amortization									85.8	77.4	63.9
Asset impairment									0
Other operating expenses									0.6	2.5	3.3
Total Operating Expenses									172.0	154.5	139.3
Net gain (loss) from asset sales									(0.2)	2.4	(0.1)
OPERATING INCOME									167.7	158.1	145.7
Interest and other income									5.0	2.8	4.2
Income from unconsolidated affiliate									0	0	0
Interest expense									(0.1)	0	0
Income taxes									(62.0)	(57.0)	(54.7)
NET INCOME									110.6	103.9	95.2
Additional Line-of-Business Information [Abstract]
Identifiable assets	993.8				836.1				993.8	836.1	768.2
Goodwill	0				0				0	0	0
Investment in unconsolidated affiliate	0				0				0	0	0
Cash capital expenditures including acquisitions									249.5	144.5	136.2
Accrued capital expenditures including acquisitions									240.7	145.5	144.3
Operating Segments [Member] | Wexpro [Member] | From unaffiliated customers [Member]
Revenues [Abstract]
Total Revenues									45.1	36.1	31.5
Operating Segments [Member] | Wexpro [Member] | From affiliated companies [Member]
Revenues [Abstract]
Total Revenues									294.8	274.1	253.6
Operating Segments [Member] | Questar Pipeline [Member]
Revenues [Abstract]
Total Revenues									266.2	277.5	271.8
Operating Expenses [Abstract]
Cost of sales									6.1	6.7	3.1
Operating and maintenance									32.5	35.2	35.3
General and administrative									46.7	50.0	46.5
Retirement incentive										0.9
Production and other taxes									9.3	9.1	10.1
Depreciation, depletion and amortization									55.5	54.3	51.2
Asset impairment									80.6
Other operating expenses									0	0	0
Total Operating Expenses									230.7	156.2	146.2
Net gain (loss) from asset sales									0	2.7	0.3
OPERATING INCOME									35.5	124.0	125.9
Interest and other income									1.8	0.6	0.9
Income from unconsolidated affiliate									3.7	3.7	3.8
Interest expense									(25.8)	(26.3)	(24.5)
Income taxes									(7.0)	(37.3)	(38.2)
NET INCOME									8.2	64.7	67.9
Additional Line-of-Business Information [Abstract]
Identifiable assets	1,222.8				1,298.9				1,222.8	1,298.9	1,286.6
Goodwill	4.2				4.2				4.2	4.2	4.2
Investment in unconsolidated affiliate	25.6				26.5				25.6	26.5	27.3
Cash capital expenditures including acquisitions									73.4	60.6	104.5
Accrued capital expenditures including acquisitions									70.0	58.6	93.8
Operating Segments [Member] | Questar Pipeline [Member] | From unaffiliated customers [Member]
Revenues [Abstract]
Total Revenues									189.5	203.1	197.4
Operating Segments [Member] | Questar Pipeline [Member] | From affiliated companies [Member]
Revenues [Abstract]
Total Revenues									76.7	74.4	74.4
Corporate and Other [Member]
Revenues [Abstract]
Total Revenues									0.2	0	0
Operating Expenses [Abstract]
Cost of sales									0.1	0	0
Operating and maintenance									0.1	0	0
General and administrative									(6.3)	(6.6)	(3.3)
Retirement incentive										1.4
Production and other taxes									1.8	1.8	1.8
Depreciation, depletion and amortization									3.8	2.7	0.3
Asset impairment									0
Other operating expenses									0	0	0
Total Operating Expenses									(0.5)	(0.7)	(1.2)
Net gain (loss) from asset sales									0	0	0
OPERATING INCOME									0.7	0.7	1.2
Interest and other income									(1.3)	(1.2)	0.5
Income from unconsolidated affiliate									0	0	0
Interest expense									(9.4)	(8.2)	(7.0)
Income taxes									(0.4)	5.0	4.0
NET INCOME									(10.4)	(3.7)	(1.3)
Additional Line-of-Business Information [Abstract]
Identifiable assets	75.7				74.8				75.7	74.8	33.6
Goodwill	0				0				0	0	0
Investment in unconsolidated affiliate	0				0				0	0	0
Cash capital expenditures including acquisitions									14.6	3.5	5.5
Accrued capital expenditures including acquisitions									17.6	1.0	8.1
Corporate and Other [Member] | From unaffiliated customers [Member]
Revenues [Abstract]
Total Revenues									0.2	0	0
Corporate and Other [Member] | From affiliated companies [Member]
Revenues [Abstract]
Total Revenues									0	0	0
Interco. Trans. [Member]
Revenues [Abstract]
Total Revenues									(372.1)	(351.0)	(331.3)
Operating Expenses [Abstract]
Cost of sales									(370.9)	(347.7)	(327.3)
Operating and maintenance									0	(0.2)	(0.2)
General and administrative									(0.6)	(0.6)	(0.5)
Retirement incentive										0
Production and other taxes									0	0	0
Depreciation, depletion and amortization									0	0	0
Asset impairment									0
Other operating expenses									(0.6)	(2.5)	(3.3)
Total Operating Expenses									(372.1)	(351.0)	(331.3)
Net gain (loss) from asset sales									0	0	0
OPERATING INCOME									0	0	0
Interest and other income									(0.7)	(0.7)	(0.6)
Income from unconsolidated affiliate									0	0	0
Interest expense									0.7	0.7	0.6
Income taxes									0	0	0
NET INCOME									0	0	0
Additional Line-of-Business Information [Abstract]
Identifiable assets	0				0				0	0	0
Goodwill	0				0				0	0	0
Investment in unconsolidated affiliate	0				0				0	0	0
Cash capital expenditures including acquisitions									0	0	0
Accrued capital expenditures including acquisitions									0	0	0
Interco. Trans. [Member] | From unaffiliated customers [Member]
Revenues [Abstract]
Total Revenues									0	0	0
Interco. Trans. [Member] | From affiliated companies [Member]
Revenues [Abstract]
Total Revenues									$ (372.1)	$ (351.0)	$ (331.3)