Accrued Liabilities (Accrued Liabilities) (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Accounts Payable and Accrued Liabilities, Current [Abstract]
Employee costs	$ 19,269	$ 24,632
Lease operating expenses	12,612	10,903
Taxes	9,507	7,308
Interest payable	7,024	6,568
Other	10,420	4,687
Total accrued liabilities	$ 58,832	$ 54,098