Organization and Accounting Policies (Reconciliation of Cash, Cash Equivalents, and Restricted Cash) (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018	Jun. 30, 2018	Dec. 31, 2017
Organization and Accounting Policies [Abstract]
Cash and cash equivalents	$ 48,557	$ 33,360
Restricted cash - current	799	804
Restricted cash - non-current	922	920
Abandonment funding	11,550	11,571
Total cash, cash equivalents and restricted cash shown in the condensed consolidated statements of cash flows	$ 61,828	$ 46,655	$ 53,245	$ 32,286