Accrued Liabilities (Accrued Liabilities) (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Accrued Liabilities [Abstract]
Employee costs	$ 13,910	$ 22,056
Lease operating expenses	10,552	12,756
Interest payable	6,741	6,635
Taxes	6,675	1,378
Third-party credits	2,824	2,129
Other	4,473	4,710
Total accrued liabilities	$ 45,175	$ 49,664