Organization and Accounting Policies (Reconciliation of Cash, Cash Equivalents, and Restricted Cash) (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020	Sep. 30, 2020	Dec. 31, 2019
Organization and Accounting Policies [Abstract]
Cash and cash equivalents	$ 52,839	$ 47,853	$ 41,986
Restricted cash - current	81	86	82
Restricted cash - non-current	1,752	925	925
Abandonment funding	22,281	12,453	11,885
Total cash, cash equivalents and restricted cash	$ 76,953	$ 61,317	$ 54,878	$ 59,124