Accrued Liabilities (Accrued Liabilities) (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Accrued Liabilities [Abstract]
Employee costs	$ 12,359	$ 19,521
Lease operating expenses	12,080	11,819
Taxes	6,997	3,404
Interest payable	6,581	6,745
Derivative settlements	2,550	0
Third-party credits	2,473	2,240
Other	6,024	4,794
Total accrued liabilities	$ 49,064	$ 48,523