Operations by Line of Business (Details) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2015 USD ($)	Sep. 30, 2015 USD ($)	Jun. 30, 2015 USD ($)	Mar. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Sep. 30, 2014 USD ($)	Jun. 30, 2014 USD ($)	Mar. 31, 2014 USD ($)	Dec. 31, 2015 USD ($) lines_of_business	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)
Segment Reporting Information [Line Items]
Number of complementary lines of business | lines_of_business									3
Revenues [Abstract]
Total Revenues	$ 364.7	$ 142.3	$ 199.3	$ 428.6	$ 373.2	$ 157.9	$ 201.3	$ 456.9	$ 1,134.9	$ 1,189.3	$ 1,220.0
Operating Expenses [Abstract]
Cost of sales									175.2	186.3	285.9
Operating and maintenance									181.6	194.2	179.4
General and administrative									109.0	122.7	115.9
Pension settlement costs	16.7	0.0	0.0	0.0					16.7	0.0	0.0
Production and other taxes									51.3	66.2	57.4
Depreciation, depletion and amortization									216.0	213.7	194.8
Abandonment and impairment									12.5	2.0	80.6
Other operating expenses											0.0
Total Operating Expenses									762.3	785.1	914.0
Net gain (loss) from asset sales									1.8	1.2	(0.2)
Operating Income (Loss)	89.8	62.3	75.3	147.0	111.7	68.9	76.5	148.3	374.4	405.4	305.8
Interest and other income									4.2	6.6	9.9
Income from unconsolidated affiliate									3.7	3.5	3.7
Interest expense									(63.0)	(63.1)	(56.9)
Income taxes									(110.6)	(125.9)	(101.3)
Net Income (Loss)	50.9	$ 32.6	$ 40.6	$ 84.6	62.5	$ 38.6	$ 40.3	$ 85.1	208.7	226.5	161.2
Additional Line-of-Business Information [Abstract]
Identifiable assets	4,377.8				4,243.9				4,377.8	4,243.9	4,044.6
Goodwill	9.8				9.8				9.8	9.8	9.8
Investment in unconsolidated affiliate	23.9				24.7				23.9	24.7	25.6
Cash capital expenditures, including acquisitions									330.4	371.5	503.7
Accrued capital expenditures, including acquisitions									328.0	359.7	505.6
From unaffiliated customers [Member]
Revenues [Abstract]
Total Revenues									1,134.9	1,189.3	1,220.0
From affiliated companies [Member]
Revenues [Abstract]
Total Revenues									0.0	0.0	0.0
Operating Segments [Member] | Questar Gas [Member]
Revenues [Abstract]
Total Revenues									917.6	960.9	985.8
Operating Expenses [Abstract]
Cost of sales									558.1	604.8	650.6
Operating and maintenance									111.9	122.5	113.1
General and administrative									50.6	52.8	52.5
Pension settlement costs									0.0
Production and other taxes									19.3	17.8	18.0
Depreciation, depletion and amortization									55.1	53.6	49.7
Abandonment and impairment									0.0	0.0	0.0
Other operating expenses											0.0
Total Operating Expenses									795.0	851.5	883.9
Net gain (loss) from asset sales									0.0	0.1	0.0
Operating Income (Loss)									122.6	109.5	101.9
Interest and other income									4.8	5.9	5.1
Income from unconsolidated affiliate									0.0	0.0	0.0
Interest expense									(28.3)	(28.2)	(22.3)
Income taxes									(34.8)	(32.0)	(31.9)
Net Income (Loss)									64.3	55.2	52.8
Additional Line-of-Business Information [Abstract]
Identifiable assets	2,126.8				1,917.4				2,126.8	1,917.4	1,759.1
Goodwill	5.6				5.6				5.6	5.6	5.6
Investment in unconsolidated affiliate	0.0				0.0				0.0	0.0	0.0
Cash capital expenditures, including acquisitions									228.8	174.7	166.2
Accrued capital expenditures, including acquisitions									232.6	161.5	177.3
Operating Segments [Member] | Questar Gas [Member] | From unaffiliated customers [Member]
Revenues [Abstract]
Total Revenues									917.6	960.9	985.2
Operating Segments [Member] | Questar Gas [Member] | From affiliated companies [Member]
Revenues [Abstract]
Total Revenues									0.0	0.0	0.6
Operating Segments [Member] | Wexpro [Member]
Revenues [Abstract]
Total Revenues									342.0	385.9	339.9
Operating Expenses [Abstract]
Cost of sales									0.0	0.0	0.0
Operating and maintenance									29.6	31.5	28.6
General and administrative									33.5	32.6	28.7
Pension settlement costs									0.0
Production and other taxes									21.2	37.6	28.3
Depreciation, depletion and amortization									99.4	100.5	85.8
Abandonment and impairment									12.5	2.0	0.0
Other operating expenses											0.6
Total Operating Expenses									196.2	204.2	172.0
Net gain (loss) from asset sales									1.6	1.6	(0.2)
Operating Income (Loss)									147.4	183.3	167.7
Interest and other income									0.9	1.1	5.0
Income from unconsolidated affiliate									0.0	0.0	0.0
Interest expense									(0.2)	(0.1)	(0.1)
Income taxes									(49.2)	(61.5)	(62.0)
Net Income (Loss)									98.9	122.8	110.6
Additional Line-of-Business Information [Abstract]
Identifiable assets	920.6				970.6				920.6	970.6	988.3
Goodwill	0.0				0.0				0.0	0.0	0.0
Investment in unconsolidated affiliate	0.0				0.0				0.0	0.0	0.0
Cash capital expenditures, including acquisitions									46.4	114.4	249.5
Accrued capital expenditures, including acquisitions									42.9	114.7	240.7
Operating Segments [Member] | Wexpro [Member] | From unaffiliated customers [Member]
Revenues [Abstract]
Total Revenues									22.9	35.6	45.1
Operating Segments [Member] | Wexpro [Member] | From affiliated companies [Member]
Revenues [Abstract]
Total Revenues									319.1	350.3	294.8
Operating Segments [Member] | Questar Pipeline [Member]
Revenues [Abstract]
Total Revenues									263.0	263.9	266.2
Operating Expenses [Abstract]
Cost of sales									8.9	4.0	6.1
Operating and maintenance									37.6	39.3	37.6
General and administrative									38.9	38.9	41.6
Pension settlement costs									0.0
Production and other taxes									8.8	9.1	9.3
Depreciation, depletion and amortization									54.6	54.5	55.5
Abandonment and impairment									0.0	0.0	80.6
Other operating expenses											0.0
Total Operating Expenses									148.8	145.8	230.7
Net gain (loss) from asset sales									0.2	(0.5)	0.0
Operating Income (Loss)									114.4	117.6	35.5
Interest and other income									0.9	1.2	1.8
Income from unconsolidated affiliate									3.7	3.5	3.7
Interest expense									(26.0)	(26.1)	(25.8)
Income taxes									(33.4)	(35.6)	(7.0)
Net Income (Loss)									59.6	60.6	8.2
Additional Line-of-Business Information [Abstract]
Identifiable assets	1,227.3				1,230.5				1,227.3	1,230.5	1,221.0
Goodwill	4.2				4.2				4.2	4.2	4.2
Investment in unconsolidated affiliate	23.9				24.7				23.9	24.7	25.6
Cash capital expenditures, including acquisitions									40.6	59.4	73.4
Accrued capital expenditures, including acquisitions									39.5	61.3	70.0
Operating Segments [Member] | Questar Pipeline [Member] | From unaffiliated customers [Member]
Revenues [Abstract]
Total Revenues									187.9	190.2	189.5
Operating Segments [Member] | Questar Pipeline [Member] | From affiliated companies [Member]
Revenues [Abstract]
Total Revenues									75.1	73.7	76.7
Corporate and Other [Member]
Revenues [Abstract]
Total Revenues									6.5	2.6	0.2
Operating Expenses [Abstract]
Cost of sales									1.7	0.9	0.1
Operating and maintenance									2.5	0.9	0.1
General and administrative									(13.3)	(1.0)	(6.3)
Pension settlement costs									16.7
Production and other taxes									2.0	1.7	1.8
Depreciation, depletion and amortization									6.9	5.1	3.8
Abandonment and impairment									0.0	0.0	0.0
Other operating expenses											0.0
Total Operating Expenses									16.5	7.6	(0.5)
Net gain (loss) from asset sales									0.0	0.0	0.0
Operating Income (Loss)									(10.0)	(5.0)	0.7
Interest and other income									(1.4)	(1.2)	(1.3)
Income from unconsolidated affiliate									0.0	0.0	0.0
Interest expense									(9.5)	(9.1)	(9.4)
Income taxes									6.8	3.2	(0.4)
Net Income (Loss)									(14.1)	(12.1)	(10.4)
Additional Line-of-Business Information [Abstract]
Identifiable assets	103.1				125.4				103.1	125.4	76.2
Goodwill	$ 0.0				0.0				$ 0.0	0.0	0.0
Investment in unconsolidated affiliate					0.0					0.0	0.0
Cash capital expenditures, including acquisitions									$ 14.6	23.0	14.6
Accrued capital expenditures, including acquisitions									13.0	22.2	17.6
Corporate and Other [Member] | From unaffiliated customers [Member]
Revenues [Abstract]
Total Revenues									6.5	2.6	0.2
Corporate and Other [Member] | From affiliated companies [Member]
Revenues [Abstract]
Total Revenues									0.0	0.0	0.0
Interco. Trans. [Member]
Revenues [Abstract]
Total Revenues									(394.2)	(424.0)	(372.1)
Operating Expenses [Abstract]
Cost of sales									(393.5)	(423.4)	(370.9)
Operating and maintenance									0.0	0.0	0.0
General and administrative									(0.7)	(0.6)	(0.6)
Pension settlement costs									0.0
Production and other taxes									0.0	0.0	0.0
Depreciation, depletion and amortization									0.0	0.0	0.0
Abandonment and impairment									0.0	0.0	0.0
Other operating expenses											(0.6)
Total Operating Expenses									(394.2)	(424.0)	(372.1)
Net gain (loss) from asset sales									0.0	0.0	0.0
Operating Income (Loss)									0.0	0.0	0.0
Interest and other income									(1.0)	(0.4)	(0.7)
Income from unconsolidated affiliate									0.0	0.0	0.0
Interest expense									1.0	0.4	0.7
Income taxes									0.0	0.0	0.0
Net Income (Loss)									0.0	0.0	0.0
Additional Line-of-Business Information [Abstract]
Identifiable assets	$ 0.0				0.0				0.0	0.0	0.0
Goodwill	0.0				0.0				0.0	0.0	0.0
Investment in unconsolidated affiliate	$ 0.0				$ 0.0				0.0	0.0	0.0
Cash capital expenditures, including acquisitions									0.0	0.0	0.0
Accrued capital expenditures, including acquisitions									0.0	0.0	0.0
Interco. Trans. [Member] | From unaffiliated customers [Member]
Revenues [Abstract]
Total Revenues									0.0	0.0	0.0
Interco. Trans. [Member] | From affiliated companies [Member]
Revenues [Abstract]
Total Revenues									$ (394.2)	$ (424.0)	$ (372.1)