Supplemental Financial Information Exploration and Production (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Capitalized Costs Relating to Oil and Gas Producing Activities, by Geographic Area [Line Items]
Capitalized costs, oil and gas producing activities, subject to amortization	$ 2,345,114	$ 2,138,565	$ 1,815,380
Capitalized Costs of Unproved Properties Excluded from Amortization, Cumulative	232,462	182,402	178,214
Capitalized Costs, Oil and Gas Producing Activities, Gross	2,577,576	2,320,967	1,993,594
Capitalized Costs, Accumulated Depreciation, Depletion, Amortization and Valuation Allowance Relating to Oil and Gas Producing Activities	1,229,654	1,093,723	969,630
Capitalized Costs, Oil and Gas Producing Activities, Net	$ 1,347,922	$ 1,227,244	$ 1,023,964