Assets Held For Sale (Details) (USD $)	12 Months Ended				0 Months Ended
	Dec. 31, 2011		Dec. 31, 2010	Dec. 31, 2009	Dec. 21, 2011 Nevada Assets	Dec. 22, 2011 Nevada Assets	Dec. 31, 2011 Nevada Assets
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations
Assets held-for-sale related to proved developed properties	$ 14,622,000		$ 0
Subsequent event, amount					16,500,000
Deposits on asset sales	(3,300,000)		0	0		(3,300,000)
Asset retirement obligations	$ 64,019,000	[1]	$ 53,443,000	$ 43,487,000			$ 700,000

[1] The asset retirement obligation related to the Nevada Assets, which were held for sale as of December 31, 2011, was $0.7 million at December 31, 2011 and is included in the asset retirement obligations liability on the December 31, 2011 Balance Sheets. See Note 3 to the Financial Statements.