Accrued Liabilities (Accrued Liabilities) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Accrued Liabilities [Abstract]
Employee costs	$ 19,521	$ 15,394
Lease operating expenses	11,819	10,075
Interest payable	6,745	6,524
Taxes	3,404	2,219
Third-party credits	2,240	2,998
Other	4,794	2,441
Total accrued liabilities	$ 48,523	$ 39,651