Supplemental Gas and Oil Information Results of Operations for Oil and Gas Producing Activities (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Results of Operations for Cost-of-Service Oil and Gas Producing Activities [Line Items]
Cost incurred for cost-of-service gas and oil producing activities	$ 149.1	$ 87.0	$ 113.2
Cost incurred to develop proved undeveloped reserves	69.7	8.5	27.7
Revenues [Abstract]
From unaffiliated customers	31.5	24.6	17.8
From affiliated company	253.6	240.2	225.1
Total revenues	285.1	264.8	242.9
Production costs	51.2	48.1	42.1
Depreciation, depletion and amortization	63.9	62.1	58.8
Total expenses	115.1	110.2	100.9
Revenues less expenses	170.0	154.6	142.0
Income Taxes	(62.0)	(54.9)	(51.7)
Results of operations for cost-of-service gas- and oil-producing activities excluding corporate overhead and interest expenses	108.0	99.7	90.3
Reimbursement of general and administrative expenses from Questar Gas	$ 23.5	$ 19.8	$ 16.7