Organization and Accounting Policies (Reconciliation of Cash, Cash Equivalents, and Restricted Cash) (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020	Mar. 31, 2020	Dec. 31, 2019
Organization and Accounting Policies [Abstract]
Cash and cash equivalents	$ 19,251	$ 47,853	$ 60,973
Restricted cash - current	82	86	994
Restricted cash - non-current	1,752	925	925
Abandonment funding	22,862	12,453	11,227
Total cash, cash equivalents and restricted cash	$ 43,947	$ 61,317	$ 74,119	$ 59,124