Derivative Instruments and Hedging Activities (Details) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010
Derivative Instruments, Gain (Loss) [Line Items]
Ineffectiveness on interest rate swaps	$ 0	$ (683,000)	$ 0	$ (870,000)
Derivatives expense (income)	(172,904,000)	(128,674,000)	(2,154,000)	(169,899,000)
Oil contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Payment (receipt) on settlements of derivative contracts	16,972,000	13,829,000	22,000,000	77,379,000
Fair value adjustments to derivative contracts - expense (income)	(187,194,000)	(145,099,000)	(20,130,000)	(206,920,000)
Total derivatives expense (income) - oil & natural gas	(170,222,000)	(131,270,000)	1,870,000	(129,541,000)
Natural Gas Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Payment (receipt) on settlements of derivative contracts	(6,030,000)	(16,630,000)	(12,646,000)	(20,379,000)
Fair value adjustments to derivative contracts - expense (income)	3,348,000	19,909,000	8,622,000	(19,109,000)
Total derivatives expense (income) - oil & natural gas	$ (2,682,000)	$ 3,279,000	$ (4,024,000)	$ (39,488,000)