The cash outflow associated with the acquisition of long-lived, physical assets and cash payments to unconsolidated affiliates that are used in the normal conduct of business to produce goods and services and not intended for resale; includes cash outflows to pay for construction of self-constructed assets.

No definition available.

Operations by Line of Business (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014 lines_of_business	Dec. 31, 2013	Dec. 31, 2012
Segment Reporting Information [Line Items]
Number of complementary lines of business									3
Revenues [Abstract]
Total Revenues	$ 373.2us-gaap_Revenues	$ 157.9us-gaap_Revenues	$ 201.3us-gaap_Revenues	$ 456.9us-gaap_Revenues	$ 397.9us-gaap_Revenues	$ 149.6us-gaap_Revenues	$ 195.6us-gaap_Revenues	$ 476.9us-gaap_Revenues	$ 1,189.3us-gaap_Revenues	$ 1,220.0us-gaap_Revenues	$ 1,098.9us-gaap_Revenues
Operating Expenses [Abstract]
Cost of sales									186.3us-gaap_CostOfGoodsSoldOilAndGas	285.9us-gaap_CostOfGoodsSoldOilAndGas	192.3us-gaap_CostOfGoodsSoldOilAndGas
Operating and maintenance									189.0	174.3	180.8
General and administrative									127.9us-gaap_GeneralAndAdministrativeExpense	121.0us-gaap_GeneralAndAdministrativeExpense	120.8us-gaap_GeneralAndAdministrativeExpense
Retirement incentive									0us-gaap_OtherRestructuringCosts	0us-gaap_OtherRestructuringCosts	4.9us-gaap_OtherRestructuringCosts
Production and other taxes									66.2	57.4	47.9
Depreciation, depletion and amortization									213.7us-gaap_DepreciationAndAmortization	194.8us-gaap_DepreciationAndAmortization	181.6us-gaap_DepreciationAndAmortization
Abandonment and impairment					0us-gaap_AssetImpairmentCharges	80.6us-gaap_AssetImpairmentCharges	0us-gaap_AssetImpairmentCharges	0us-gaap_AssetImpairmentCharges	2.0us-gaap_AssetImpairmentCharges	80.6us-gaap_AssetImpairmentCharges	0us-gaap_AssetImpairmentCharges
Other operating expenses										0us-gaap_OtherCostAndExpenseOperating	0us-gaap_OtherCostAndExpenseOperating
Total Operating Expenses									785.1us-gaap_OperatingExpenses	914.0us-gaap_OperatingExpenses	728.3us-gaap_OperatingExpenses
Net gain (loss) from asset sales									1.2	(0.2)	5.1
Operating Income (Loss)	111.7us-gaap_OperatingIncomeLoss	68.9us-gaap_OperatingIncomeLoss	76.5us-gaap_OperatingIncomeLoss	148.3us-gaap_OperatingIncomeLoss	122.6us-gaap_OperatingIncomeLoss	(19.4)us-gaap_OperatingIncomeLoss	71.2us-gaap_OperatingIncomeLoss	131.4us-gaap_OperatingIncomeLoss	405.4us-gaap_OperatingIncomeLoss	305.8us-gaap_OperatingIncomeLoss	375.7us-gaap_OperatingIncomeLoss
Interest and other income									6.6us-gaap_InvestmentIncomeNonoperating	9.9us-gaap_InvestmentIncomeNonoperating	7.0us-gaap_InvestmentIncomeNonoperating
Income from unconsolidated affiliate									3.5	3.7	3.7
Interest expense									(63.1)us-gaap_InterestExpense	(56.9)us-gaap_InterestExpense	(57.9)us-gaap_InterestExpense
Income taxes									(125.9)us-gaap_IncomeTaxExpenseBenefit	(101.3)us-gaap_IncomeTaxExpenseBenefit	(116.5)us-gaap_IncomeTaxExpenseBenefit
Net Income (Loss)	62.5us-gaap_NetIncomeLoss	38.6us-gaap_NetIncomeLoss	40.3us-gaap_NetIncomeLoss	85.1us-gaap_NetIncomeLoss	68.1us-gaap_NetIncomeLoss	(19.2)us-gaap_NetIncomeLoss	39.4us-gaap_NetIncomeLoss	72.9us-gaap_NetIncomeLoss	226.5us-gaap_NetIncomeLoss	161.2us-gaap_NetIncomeLoss	212.0us-gaap_NetIncomeLoss
Additional Line-of-Business Information [Abstract]
Identifiable assets	4,249.7us-gaap_Assets				4,054.3us-gaap_Assets				4,249.7us-gaap_Assets	4,054.3us-gaap_Assets	3,803.3us-gaap_Assets
Goodwill	9.8us-gaap_Goodwill				9.8us-gaap_Goodwill				9.8us-gaap_Goodwill	9.8us-gaap_Goodwill	9.8us-gaap_Goodwill
Investment in unconsolidated affiliate	24.7us-gaap_EquityMethodInvestments				25.6us-gaap_EquityMethodInvestments				24.7us-gaap_EquityMethodInvestments	25.6us-gaap_EquityMethodInvestments	26.5us-gaap_EquityMethodInvestments
Cash capital expenditures, including acquisitions									371.5	503.7	370.7
Accrued capital expenditures, including acquisitions									359.7	505.6	366.3
From unaffiliated customers [Member]
Revenues [Abstract]
Total Revenues									1,189.3	1,220.0	1,098.9
From affiliated companies [Member]
Revenues [Abstract]
Total Revenues									0	0	0
Operating Segments [Member] \| Questar Gas [Member]
Revenues [Abstract]
Total Revenues									960.9	985.8	862.2
Operating Expenses [Abstract]
Cost of sales									604.8	650.6	533.3
Operating and maintenance									122.5	113.1	119.0
General and administrative									52.8	52.5	51.2
Retirement incentive											2.4
Production and other taxes									17.8	18.0	16.2
Depreciation, depletion and amortization									53.6	49.7	47.2
Abandonment and impairment									0	0
Other operating expenses										0	0
Total Operating Expenses									851.5	883.9	769.3
Net gain (loss) from asset sales									0.1	0	0
Operating Income (Loss)									109.5	101.9	92.9
Interest and other income									5.9	5.1	5.5
Income from unconsolidated affiliate									0	0	0
Interest expense									(28.2)	(22.3)	(24.1)
Income taxes									(32.0)	(31.9)	(27.2)
Net Income (Loss)									55.2	52.8	47.1
Additional Line-of-Business Information [Abstract]
Identifiable assets	1,911.1				1,762.0				1,911.1	1,762.0	1,593.5
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									174.7	166.2	162.1
Accrued capital expenditures, including acquisitions									161.5	177.3	161.2
Operating Segments [Member] \| Questar Gas [Member] \| From unaffiliated customers [Member]
Revenues [Abstract]
Total Revenues									960.9	985.2	859.7
Operating Segments [Member] \| Questar Gas [Member] \| From affiliated companies [Member]
Revenues [Abstract]
Total Revenues									0	0.6	2.5
Operating Segments [Member] \| Wexpro [Member]
Revenues [Abstract]
Total Revenues									385.9	339.9	310.2
Operating Expenses [Abstract]
Cost of sales									0	0	0
Operating and maintenance									31.5	28.6	26.8
General and administrative									32.6	28.7	26.8
Retirement incentive											0.2
Production and other taxes									37.6	28.3	20.8
Depreciation, depletion and amortization									100.5	85.8	77.4
Abandonment and impairment									2.0	0
Other operating expenses										0.6	2.5
Total Operating Expenses									204.2	172.0	154.5
Net gain (loss) from asset sales									1.6	(0.2)	2.4
Operating Income (Loss)									183.3	167.7	158.1
Interest and other income									1.1	5.0	2.8
Income from unconsolidated affiliate									0	0	0
Interest expense									(0.1)	(0.1)	0
Income taxes									(61.5)	(62.0)	(57.0)
Net Income (Loss)									122.8	110.6	103.9
Additional Line-of-Business Information [Abstract]
Identifiable assets	977.5				993.8				977.5	993.8	836.1
Goodwill	0				0				0	0	0
Investment in unconsolidated affiliate	0				0				0	0	0
Cash capital expenditures, including acquisitions									114.4	249.5	144.5
Accrued capital expenditures, including acquisitions									114.7	240.7	145.5
Operating Segments [Member] \| Wexpro [Member] \| From unaffiliated customers [Member]
Revenues [Abstract]
Total Revenues									35.6	45.1	36.1
Operating Segments [Member] \| Wexpro [Member] \| From affiliated companies [Member]
Revenues [Abstract]
Total Revenues									350.3	294.8	274.1
Operating Segments [Member] \| Questar Pipeline [Member]
Revenues [Abstract]
Total Revenues									263.9	266.2	277.5
Operating Expenses [Abstract]
Cost of sales									4.0	6.1	6.7
Operating and maintenance									34.1	32.5	35.2
General and administrative									44.1	46.7	50.0
Retirement incentive											0.9
Production and other taxes									9.1	9.3	9.1
Depreciation, depletion and amortization									54.5	55.5	54.3
Abandonment and impairment									0	80.6
Other operating expenses										0	0
Total Operating Expenses									145.8	230.7	156.2
Net gain (loss) from asset sales									(0.5)	0	2.7
Operating Income (Loss)									117.6	35.5	124.0
Interest and other income									1.2	1.8	0.6
Income from unconsolidated affiliate									3.5	3.7	3.7
Interest expense									(26.1)	(25.8)	(26.3)
Income taxes									(35.6)	(7.0)	(37.3)
Net Income (Loss)									60.6	8.2	64.7
Additional Line-of-Business Information [Abstract]
Identifiable assets	1,232.4				1,222.8				1,232.4	1,222.8	1,298.9
Goodwill	4.2				4.2				4.2	4.2	4.2
Investment in unconsolidated affiliate	24.7				25.6				24.7	25.6	26.5
Cash capital expenditures, including acquisitions									59.4	73.4	60.6
Accrued capital expenditures, including acquisitions									61.3	70.0	58.6
Operating Segments [Member] \| Questar Pipeline [Member] \| From unaffiliated customers [Member]
Revenues [Abstract]
Total Revenues									190.2	189.5	203.1
Operating Segments [Member] \| Questar Pipeline [Member] \| From affiliated companies [Member]
Revenues [Abstract]
Total Revenues									73.7	76.7	74.4
Corporate and Other [Member]
Revenues [Abstract]
Total Revenues									2.6	0.2	0
Operating Expenses [Abstract]
Cost of sales									0.9	0.1	0
Operating and maintenance									0.9	0.1	0
General and administrative									(1.0)	(6.3)	(6.6)
Retirement incentive											1.4
Production and other taxes									1.7	1.8	1.8
Depreciation, depletion and amortization									5.1	3.8	2.7
Abandonment and impairment									0	0
Other operating expenses										0	0
Total Operating Expenses									7.6	(0.5)	(0.7)
Net gain (loss) from asset sales									0	0	0
Operating Income (Loss)									(5.0)	0.7	0.7
Interest and other income									(1.2)	(1.3)	(1.2)
Income from unconsolidated affiliate									0	0	0
Interest expense									(9.1)	(9.4)	(8.2)
Income taxes									3.2	(0.4)	5.0
Net Income (Loss)									(12.1)	(10.4)	(3.7)
Additional Line-of-Business Information [Abstract]
Identifiable assets	128.7				75.7				128.7	75.7	74.8
Goodwill	0				0				0	0	0
Investment in unconsolidated affiliate	0				0				0	0	0
Cash capital expenditures, including acquisitions									23.0	14.6	3.5
Accrued capital expenditures, including acquisitions									22.2	17.6	1.0
Corporate and Other [Member] \| From unaffiliated customers [Member]
Revenues [Abstract]
Total Revenues									2.6	0.2	0
Corporate and Other [Member] \| From affiliated companies [Member]
Revenues [Abstract]
Total Revenues									0	0	0
Interco. Trans. [Member]
Revenues [Abstract]
Total Revenues									(424.0)	(372.1)	(351.0)
Operating Expenses [Abstract]
Cost of sales									(423.4)	(370.9)	(347.7)
Operating and maintenance									0	0	(0.2)
General and administrative									(0.6)	(0.6)	(0.6)
Retirement incentive											0
Production and other taxes									0	0	0
Depreciation, depletion and amortization									0	0	0
Abandonment and impairment									0	0
Other operating expenses										(0.6)	(2.5)
Total Operating Expenses									(424.0)	(372.1)	(351.0)
Net gain (loss) from asset sales									0	0	0
Operating Income (Loss)									0	0	0
Interest and other income									(0.4)	(0.7)	(0.7)
Income from unconsolidated affiliate									0	0	0
Interest expense									0.4	0.7	0.7
Income taxes									0	0	0
Net Income (Loss)									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									$ (424.0)	$ (372.1)	$ (351.0)