Accrued Liabilities (Accrued Liabilities) (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Accrued Liabilities [Abstract]
Employee costs	$ 17,880	$ 19,521
Interest payable	17,446	6,745
Lease operating expenses	11,474	11,819
Taxes	10,317	3,404
Derivative settlements	3,383	0
Third-party credits	2,099	2,240
Other	5,144	4,794
Total accrued liabilities	$ 67,743	$ 48,523