Oil and Gas Properties (Details) (USD $)	6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Dec. 31, 2010
Developed Properties [Abstract]
Capitalized Costs, Proved Properties	$ 5,241,709,000		$ 4,575,222,000
Capitalized Costs, Accumulated Depreciation, Depletion, Amortization and Valuation Allowance for Relating to Oil and Gas Producing Activities	(2,134,743,000)		(1,985,799,000)
Proved	3,106,966,000		2,589,423,000
Capitalized Costs of Unproved Properties Excluded from Amortization [Abstract]
Oil Gas Properties Using Full Cost Method Accounting Unproved	524,307,000		486,247,000
Capitalized Costs, Oil and Gas Producing Activities, Net	3,631,273,000		3,075,670,000
Interest Expense, Borrowings	45,700,000	30,500,000
Interest Costs, Capitalized During Period	$ 15,500,000	$ 7,400,000