Derivative Instruments (Details 4) (USD $)	0 Months Ended	3 Months Ended		6 Months Ended
	Jun. 30, 2012 counterparty	Jun. 30, 2012 counterparty	Jun. 30, 2011	Jun. 30, 2012 counterparty	Jun. 30, 2011
Location and amount of derivative instrument (gains) losses before income taxes recognized in the condensed statements of operations
Other Comprehensive Income (Loss), Reclassification Adjustment on Derivatives Included in Net Income, before Tax		$ (1,600,000)		$ (3,700,000)
(Gain) loss recognized in earnings (cash settlements and mark-to-market movements)		(113,082,000)	(91,808,000)	(84,601,000)	35,708,000
Credit risk
Maximum loss if all counterparties to derivative contracts failed to perform	48,700,000
Number of counterparties to the Company's commodity derivative contracts (in counterparties)	9	9		9
Derivative instruments, number of largest counterparties (in counterparties)	3	3		3
Derivative value held by largest counterparties (as a percent)	69.00%	69.00%		69.00%
Oil and natural gas sales | Commodity contract
Location and amount of derivative instrument (gains) losses before income taxes recognized in the condensed statements of operations
Other Comprehensive Income (Loss), Reclassification Adjustment on Derivatives Included in Net Income, before Tax		(2,500,000)	(14,900,000)	(5,200,000)	(29,500,000)
Realized and unrealized (gain) loss on derivatives, net | Commodity contract
Location and amount of derivative instrument (gains) losses before income taxes recognized in the condensed statements of operations
(Gain) loss recognized in earnings (cash settlements and mark-to-market movements)		(113,082,000)	(91,808,000)	(84,601,000)	35,708,000
Interest | Interest rate contract
Location and amount of derivative instrument (gains) losses before income taxes recognized in the condensed statements of operations
Other Comprehensive Income (Loss), Reclassification Adjustment on Derivatives Included in Net Income, before Tax		$ 900,000	$ (400,000)	$ 1,500,000	$ (1,200,000)