Asset Retirement Obligations (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2011	Dec. 31, 2010
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
ARO liability beginning balance	$ 60.9	$ 65.0
Accretion	3.0	2.9
Liabilities incurred	1.5	1.3
Revisions	0.8	(8.3)
Liabilities settled	(2.4)	0
ARO liability ending balance	63.8	60.9
Asset Retirement Obligations By Line Of Business [Abstract]
Asset Retirement Obligations for the line of business	63.8	60.9
Wexpro [Member]
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
ARO liability ending balance	60.3	56.0
Asset Retirement Obligations By Line Of Business [Abstract]
Asset Retirement Obligations for the line of business	60.3	56.0
Questar Pipeline [Member]
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
ARO liability ending balance	2.8	4.1
Asset Retirement Obligations By Line Of Business [Abstract]
Asset Retirement Obligations for the line of business	2.8	4.1
Questar Gas [Member]
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
ARO liability ending balance	0.7	0.8
Asset Retirement Obligations By Line Of Business [Abstract]
Asset Retirement Obligations for the line of business	$ 0.7	$ 0.8

- Definition

Amount of accretion expense recognized in the income statement during the period that is associated with asset retirement obligations. Accretion expense measures and incorporates changes due to the passage of time into the carrying amount of the liability. Such accretion expense representing changes in the liability for an asset retirement obligation due to passage of time is the result of applying an interest method of allocation to the amount of the liability at the beginning of the period. The interest rate used to measure that change shall be the credit-adjusted risk-free rate that existed when the liability, or portion thereof, was initially measured. That amount shall be recognized as an increase in the carrying amount of the liability and as an expense classified as an operating item in the statement of income. Accretion expense shall not be considered to be interest cost for purposes of capitalization of interest.

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