Derivative Instruments and Hedging Activities (Summary of Derivative Income/Expense) (Details) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Derivative Instruments, Gain (Loss) [Line Items]
Derivatives income	$ (139,109,000)	$ (172,904,000)	$ (93,834,000)	$ (2,154,000)
Oil contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Payment (receipt) on settlements of derivative contracts	709,000	16,972,000	8,939,000	22,000,000
Fair value adjustments to derivative contracts - expense (income)	(140,923,000)	(187,194,000)	(98,478,000)	(20,130,000)
Total derivatives expense (income) - oil & natural gas	(140,214,000)	(170,222,000)	(89,539,000)	1,870,000
Natural Gas Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Payment (receipt) on settlements of derivative contracts	(7,991,000)	(6,030,000)	(15,031,000)	(12,646,000)
Fair value adjustments to derivative contracts - expense (income)	9,096,000	3,348,000	10,736,000	8,622,000
Total derivatives expense (income) - oil & natural gas	$ 1,105,000	$ (2,682,000)	$ (4,295,000)	$ (4,024,000)