Oil and Gas Properties (Details) (USD $)	9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Dec. 31, 2010
Developed Properties [Abstract]
Capitalized Costs, Proved Properties	$ 5,629,692,000		$ 4,575,222,000
Capitalized Costs, Accumulated Depreciation, Depletion, Amortization and Valuation Allowance for Relating to Oil and Gas Producing Activities	(2,218,156,000)		(1,985,799,000)
Proved	3,411,536,000		2,589,423,000
Capitalized Costs of Unproved Properties Excluded from Amortization [Abstract]
Oil Gas Properties Using Full Cost Method Accounting Unproved	526,579,000		486,247,000
Capitalized Costs, Oil and Gas Producing Activities, Net	3,938,115,000		3,075,670,000
Interest Expense, Borrowings	69,000,000	48,400,000
Interest Costs, Capitalized During Period	21,300,000	13,100,000
Interest Costs Capitalized Gathering Systems	$ 1,600,000	$ 800,000