Derivative Instruments and Hedging Activities (Details 2) (USD $)	Jun. 30, 2011	Dec. 31, 2010
Derivative financial instruments in Balance Sheet
Total derivatives not designated as hedging instruments	$ (48,074,000)	$ (70,710,000)
Oil contracts [Member] | Derivative assets-current [Member]
Derivative financial instruments in Balance Sheet
Derivative assets	1,462,000	3,050,000
Natural Gas Contracts [Member] | Derivative assets-current [Member]
Derivative financial instruments in Balance Sheet
Derivative assets	17,860,000	21,192,000
Oil contracts [Member] | Derivative assets - long - term [Member]
Derivative financial instruments in Balance Sheet
Derivative assets	11,281,000	1,301,000
Natural Gas Contracts [Member] | Derivative assets - long - term [Member]
Derivative financial instruments in Balance Sheet
Derivative assets	6,328,000	11,618,000
Oil contracts [Member] | Derivative liabilities-current [Member]
Derivative financial instruments in Balance Sheet
Derivative Liability	67,196,000	55,256,000
Natural Gas Contracts [Member] | Derivative liabilities-current [Member]
Derivative financial instruments in Balance Sheet
Derivative Liability	0	0
Derivative liabilities-current [Member] | Deferred Premiums [Member]
Derivative financial instruments in Balance Sheet
Derivative Liability	14,431,000	22,928,000
Oil contracts [Member] | Derivative liabilities-long term [Member]
Derivative financial instruments in Balance Sheet
Derivative Liability	2,228,000	25,906,000
Natural Gas Contracts [Member] | Derivative liabilities-long term [Member]
Derivative financial instruments in Balance Sheet
Derivative Liability	0	0
Derivative liabilities-long term [Member] | Deferred Premiums [Member]
Derivative financial instruments in Balance Sheet
Derivative Liability	$ 1,150,000	$ 3,781,000