Summary of Significant Accounting Policies (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Principles of Consolidation
Amount of equity contributed by Questar prior to the spin off		$ 250.0	$ 0
Revenue Recognition
Imbalance obligations	4.9	4.4
Gas and oil properties
Proved and unproved leaseholds		1,035.7	1,130.5
Impairment of long-lived assets
Impairment of long-lived assets	195.2
Capitalized Interest
Capitalized interest	3.0	3.1
Derivative Instruments
Unrealized Gain (Loss) on Derivatives	395.9
Derivative, Gain (Loss) on Derivative, Net after tax	248.6
Credit Risk
Bad debt expense associated with accounts receivable	0.2	(0.3)	0.4
Allowance for bad-debt expenses	1.7	2.3
Income Taxes
Unrecognized tax benefits - beginning of year	0	0	0
Unrecognized tax benefits - end of year		0	0
Components of basic and diluted shares used in the EPS calculation [Abstract]
Weighted-average basic common shares outstanding (in shares)	176.5	175.3	174.1
Potential number of shares issuable under the Long-term Stock Incentive Plan (in shares)	1.9	2.0	2.2
Average diluted common shares outstanding (in shares)	178.4	177.3	176.3
Share-Based Compensation
Share-based payment award terms	Stock options for participants have terms ranging from five to ten years with a majority issued with a seven year term. Options held by employees generally vest in three or four equal, annual installments. Restricted shares vest in equal installments over a specified number of years after the grant date with the majority vesting in three or four years. Non-vested restricted shares have voting and dividend rights; however, sale or transfer is restricted. At the time of the Spin-off, all outstanding options and restricted stock were bifurcated.
Internal Revenue Service (IRS) [Member]
Income Taxes
Income tax returns under examination	2010 and 2009
Open tax years	2011
State and Local Jurisdiction [Member]
Income Taxes
Open tax years	2008 and subsequent years
Southern Region [Member]
Impairment of long-lived assets
Impairment of long-lived assets	31.7
Northern Region [Member]
Impairment of long-lived assets
Impairment of long-lived assets	$ 163.5
Buildings [Member]
Property, Plant and Equipment [Line Items]
Estimated useful lives, minimum	10
Estimated useful lives, maximum	30
Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Estimated useful lives, minimum	3
Estimated useful lives, maximum	10
Service, transportation and field service equipment [Member]
Property, Plant and Equipment [Line Items]
Estimated useful lives, minimum	3
Estimated useful lives, maximum	7
Furniture and office equipment [Member]
Property, Plant and Equipment [Line Items]
Estimated useful lives, minimum	3
Estimated useful lives, maximum	7
Unitah Basin Field Services LLC [Member]
Investment in Unconsolidated Affiliates
Ownership percentage (in hundredths)	38.00%
Three Rivers Gathering LLC [Member]
Investment in Unconsolidated Affiliates
Ownership percentage (in hundredths)	50.00%