Organization and Accounting Policies (Reconciliation of Cash, Cash Equivalents, and Restricted Cash) (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020	Jun. 30, 2020	Dec. 31, 2019
Organization and Accounting Policies [Abstract]
Cash and cash equivalents	$ 22,884	$ 47,853	$ 44,841
Restricted cash - current	83	86	1,090
Restricted cash - non-current	1,752	925	925
Abandonment funding	22,837	12,453	11,420
Total cash, cash equivalents and restricted cash	$ 47,556	$ 61,317	$ 58,276	$ 59,124