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, 2012	Dec. 31, 2011	Dec. 31, 2010
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	$ 147.5	$ 149.1	$ 87.0
Cost incurred to develop proved undeveloped reserves	50.4	69.7	8.5
Revenues [Abstract]
From unaffiliated customers	36.1	31.5	24.6
From affiliated company	274.1	253.6	240.2
Total revenues	310.2	285.1	264.8
Production costs	50.1	51.2	48.1
Depreciation, depletion and amortization	77.4	63.9	62.1
Total expenses	127.5	115.1	110.2
Revenues less expenses	182.7	170.0	154.6
Income Taxes	(64.8)	(62.0)	(54.9)
Results of operations for cost-of-service gas- and oil-producing activities excluding corporate overhead and interest expenses	117.9	108.0	99.7
Reimbursement of general and administrative expenses from Questar Gas included in affiliated-company revenues	$ 26.5	$ 23.5	$ 19.8