Derivative Instruments and Hedging Activities (Summary of Derivative Income/Expense) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Derivative Instruments, Gain (Loss) [Line Items]
Total derivatives expense (income) - oil & natural gas	$ 11,929	$ 45,275
Derivatives expense (income)	11,929	45,275
Crude Oil Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Cash payment (receipt) on settlements of derivative contracts	0	8,230
Noncash fair value adjustments to derivative contracts - expense	11,929	42,445
Total derivatives expense (income) - oil & natural gas	11,929	50,675
Natural Gas Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Cash payment (receipt) on settlements of derivative contracts	0	(7,040)
Noncash fair value adjustments to derivative contracts - expense	0	1,640
Total derivatives expense (income) - oil & natural gas	$ 0	$ (5,400)