Derivative Instruments (Details 4) (USD $)	0 Months Ended	3 Months Ended		9 Months Ended
	Sep. 30, 2012 counterparty	Sep. 30, 2012 counterparty	Sep. 30, 2011	Sep. 30, 2012 counterparty	Sep. 30, 2011
Location and amount of derivative instrument (gains) losses before income taxes recognized in the condensed statements of operations
(Gain) loss reclassified from AOCL into earnings (amortization of frozen amount)		$ 2,400,000		$ 6,100,000
Loss (gain) recognized in earnings (cash settlements and mark-to-market movements)		28,287,000	(162,145,000)	(56,314,000)	(126,437,000)
Credit risk
Maximum loss if all counterparties to derivative contracts failed to perform	19,800,000
Number of counterparties to the Company's commodity derivative contracts (in counterparties)	9	9		9
Derivative number of counterparties with net liability position	3	3		3
Derivative instruments, number of largest counterparties (in counterparties)	2	2		2
Derivative value held by largest counterparties (as a percent)	80.00%	80.00%		80.00%
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
(Gain) loss reclassified from AOCL into earnings (amortization of frozen amount)		2,700,000	15,500,000	7,900,000	45,000,000
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
Loss (gain) recognized in earnings (cash settlements and mark-to-market movements)		28,287,000	(162,145,000)	(56,314,000)	(126,437,000)
Interest | Interest rate contract
Location and amount of derivative instrument (gains) losses before income taxes recognized in the condensed statements of operations
(Gain) loss reclassified from AOCL into earnings (amortization of frozen amount)		$ (300,000)	$ 0	$ (1,800,000)	$ 1,200,000