Accrued Liabilities (Accrued Liabilities) (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Accounts Payable and Accrued Liabilities, Current [Abstract]
Taxes	$ 22,724	$ 13,480
Employee costs	16,460	22,518
Lease operating expenses	7,877	7,346
Interest payable	3,030	2,647
Hedge settlements	0	1,844
Other	4,968	3,898
Total accrued liabilities	$ 55,059	$ 51,733