Derivative Instruments and Hedging Activities (Summary of Derivative Income/Expense) (Details) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Derivative Instruments, Gain (Loss) [Line Items]
Derivatives expense (income)	$ 61,631,000	$ (210,154,000)	$ (32,203,000)	$ (212,308,000)
Crude Oil Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Payment (receipt) on settlements of derivative contracts	641,000	1,857,000	9,580,000	23,857,000
Fair value adjustments to derivative contracts - expense (income)	60,726,000	(205,355,000)	(37,752,000)	(225,485,000)
Total derivatives expense (income) - oil & natural gas	61,367,000	(203,498,000)	(28,172,000)	(201,628,000)
Natural Gas Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Payment (receipt) on settlements of derivative contracts	(6,910,000)	(6,427,000)	(21,941,000)	(19,073,000)
Fair value adjustments to derivative contracts - expense (income)	7,174,000	(229,000)	17,910,000	8,393,000
Total derivatives expense (income) - oil & natural gas	$ 264,000	$ (6,656,000)	$ (4,031,000)	$ (10,680,000)