Derivative Instruments and Hedging Activities (Summary of Derivative Income/Expense) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Derivative Instruments, Loss (Gain) [Line Items]
Total derivatives expense (income) - oil & natural gas	$ (45,501)	$ (139,109)	$ (33,572)	$ (93,834)
Derivatives expense (income)	(45,501)	(139,109)	(33,572)	(93,834)
Crude Oil Contracts [Member]
Derivative Instruments, Loss (Gain) [Line Items]
Cash payment (receipt) on settlements of derivative contracts	0	709	0	8,939
Noncash fair value adjustments to derivative contracts - expense (income)	(45,501)	(140,923)	(33,572)	(98,478)
Total derivatives expense (income) - oil & natural gas	(45,501)	(140,214)	(33,572)	(89,539)
Natural Gas Contracts [Member]
Derivative Instruments, Loss (Gain) [Line Items]
Cash payment (receipt) on settlements of derivative contracts	0	(7,991)	0	(15,031)
Noncash fair value adjustments to derivative contracts - expense (income)	0	9,096	0	10,736
Total derivatives expense (income) - oil & natural gas	$ 0	$ 1,105	$ 0	$ (4,295)