Summary of Significant Accounting Policies (Details) (USD $)	6 Months Ended	12 Months Ended
	Jun. 30, 2010	Dec. 31, 2012 bbl lines_of_business	Dec. 31, 2011	Dec. 31, 2010
Nature of Business [Abstract]
Number Of Business Lines		3
Principles of Consolidation [Abstract]
Distribution date of tax-free, pro rata dividend to Questar shareholders		6/30/2010
Number of shares of QEP common stock received for each share of Questar common stock		1
Equity contribution to QEP prior to Spinoff	$ 250,000,000	$ 0	$ 0	$ 250,000,000
Separation costs		0	0	11,500,000
Revenue Recognition [Abstract]
After tax rate of return allowed per Wexpro agreement (In hundredths)		20.00%
Percentage of operating income retained by Wexpro after expenses (in hundredths)		46.00%
Cost of Sales [Abstract]
Total cost of sales		192,300,000	321,500,000	280,900,000
Cash and Cash Equivalents [Abstract]
Cash equivalent maturity maximum (months)		3 months
Depreciation, depletion and amortization [Abstract]
Barrel of oil conversion to 6,000 cubic feet of natural gas		1
Questar Gas distribution plant		2.80%	2.80%	2.90%
Cost-of-service gas and oil properties, per Mcfe		1.49	1.49	1.47
Questar Pipeline transportation, storage and other energy services		3.40%	3.50%	3.50%
Impairment of Long-Lived Assets [Abstract]
Total impairment expense		0	0	0
Capitalized Interest and Allowance for Funds Used During Construction [Abstract]
AFUDC (recorded as an increase in interest and other income)		2,300,000	4,400,000	2,700,000
Capitalized interest costs (recorded as a reduction of interest expense)		200,000	600,000	1,200,000
Credit Risk [Abstract]
Bad debt expense		1,200,000	2,400,000	500,000
Reduction in bad-debt expense from the settlement of a revenue dispute previously accrued for in allowance for bad debts				1,500,000
Allowance for bad debts		3,100,000	3,200,000
Income Taxes [Abstract]
Unrecognized tax benefits		0	0	0
Percent of capital expenditures deductible for federal income tax purposes under current statutes (in hundredths)		50.00%	100.00%
Estimated net operating loss (NOL) carryforward		55,100,000	124,800,000
Estimated taxable income		69,700,000
Expiration date of net operating loss (NOL) for federal income tax purposes		2031
Accumulated other comprehensive loss [Abstract]
Pension liability		(203,500,000)	(182,700,000)
Postretirement benefits liability		(19,700,000)	(20,100,000)
Interest rate cash flow hedges		(23,200,000)	(23,400,000)
Long-term investment		100,000	100,000
Accumulated other comprehensive loss		(246,300,000)	(226,100,000)
Income taxes allocated to each component of other comprehensive income (loss) [Abstract]
Change in unrecognized actuarial loss		13,600,000	62,300,000	(6,800,000)
Change in unrecognized prior-service costs		(1,000,000)	(1,100,000)	(1,400,000)
Interest rate cash flow hedges		(200,000)	13,800,000	0
Change in fair value of long-term investment		(100,000)	0	0
Discontinued operations, change in unrealized fair value of derivatives		0	0	51,600,000
Income taxes		12,300,000	75,000,000	43,400,000
Derivatives used in cash flow hedge, AOCI, net of tax [Roll Forward]
Interest rate cash flow hedges beginning balance		(23,400,000)
Interest rate cash flow hedges ending balance		(23,200,000)	(23,400,000)
Questar Gas [Member]
Revenue Recognition [Abstract]
Rate Adjustment Time Period		6 months
Deferred CET adjustments amortization period		12 months
Rate adjustment limitation as a percent of non-gas revenues (In hundredths)		5.00%
Cost of Sales [Abstract]
Gas purchases		104,200,000	221,200,000	245,400,000
Operator service fee		274,000,000	253,400,000	240,000,000
Transportation and storage		79,600,000	78,400,000	78,100,000
Gathering		20,500,000	25,000,000	23,600,000
Royalties		32,000,000	38,900,000	37,200,000
Storage (injection) withdrawal, net		1,900,000	3,000,000	(800,000)
Purchased-gas account adjustment		16,100,000	20,600,000	(36,800,000)
Other		5,000,000	5,200,000	5,500,000
Total Questar Gas cost of natural gas sold		533,300,000	645,700,000	592,200,000
Elimination of Questar Gas cost of natural gas sold - affiliated parties		(347,700,000)	(327,300,000)	(313,700,000)
Total Questar Gas cost of natural gas sold - unaffiliated parties		185,600,000	318,400,000	278,500,000
Contributions in aid of construction [Abstract]
Time period contributions are refundable if other customers pay to connect to the line (in years)		5 years
Time period if no other customers connect to the line the funds are not refundable (in years)		5 years
Capitalized Interest and Allowance for Funds Used During Construction [Abstract]
Capitalized interest costs (recorded as a reduction of interest expense)		100,000	100,000	200,000
Credit Risk [Abstract]
Time period in months uncollected accounts are written off		6 months
Wexpro [Member]
Capitalized Interest and Allowance for Funds Used During Construction [Abstract]
AFUDC (recorded as an increase in interest and other income)		2,100,000	3,300,000	2,400,000
Questar Pipeline [Member]
Investment in Unconsolidated Affiliate [Abstract]
Ownership percentage in unconsolidated affiliate (in hundredths)		50.00%
Cost of Sales [Abstract]
Total Questar Pipeline cost of sales		6,700,000	3,100,000	2,400,000
Capitalized Interest and Allowance for Funds Used During Construction [Abstract]
AFUDC (recorded as an increase in interest and other income)		200,000	1,100,000	300,000
Capitalized interest costs (recorded as a reduction of interest expense)		100,000	500,000	1,000,000
Accumulated other comprehensive loss [Abstract]
Interest rate cash flow hedges		(23,200,000)	(23,400,000)	0
Accumulated other comprehensive loss		(23,200,000)	(23,400,000)
Income taxes allocated to each component of other comprehensive income (loss) [Abstract]
Income taxes		(200,000)	13,800,000	0
Derivatives used in cash flow hedge, AOCI, net of tax [Roll Forward]
Interest rate cash flow hedges beginning balance		(23,400,000)	0
Net change associated with current period hedging transactions		0	(23,500,000)
Net amount reclassified into earnings		200,000	100,000
Interest rate cash flow hedges ending balance		$ (23,200,000)	$ (23,400,000)	$ 0