Derivative Instruments and Hedging Activities (Summary of Derivative expense (income)) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Derivative Instruments, Gain (Loss) [Line Items]
Total derivatives expense (income) - oil & natural gas			$ (23,833)
Derivatives expense (income)	(4,834)	(52,497)	(23,833)
Oil Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Payment (receipt) on settlement of derivative contracts	9,991	25,128	93,417
Fair value adjustments to derivative contracts - expense (income)	(10,904)	(58,980)	(44,441)
Total derivatives expense (income) - oil & natural gas	(913)	(33,852)	48,976
Natural Gas Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Payment (receipt) on settlement of derivative contracts	(27,871)	(27,505)	(61,805)
Fair value adjustments to derivative contracts - expense (income)	23,950	8,860	(8,585)
Total derivatives expense (income) - oil & natural gas	(3,921)	(18,645)	(70,390)
Interest Rate Swap [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Ineffectiveness on interest rate swaps	$ 0	$ 0	$ (2,419)