Accrued Liabilities (Accrued Liabilities) (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Accrued Liabilities [Abstract]
Employee costs	$ 22,056	$ 19,521
Lease operating expenses	12,756	11,819
Interest payable	6,635	6,745
Third-party credits	2,129	2,240
Taxes	1,378	3,404
Other	4,710	4,794
Total accrued liabilities	$ 49,664	$ 48,523