Summary of Significant Accounting Policies (Reconciliation of Cash, Cash Equivalents, and Restricted Cash) (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Summary of Significant Accounting Policies [Abstract]
Cash and cash equivalents	$ 47,853	$ 45,917
Restricted cash - current	86	911
Restricted cash - non-current	925	925
Abandonment funding	12,453	11,371
Total cash, cash equivalents and restricted cash	$ 61,317	$ 59,124	$ 46,655	$ 32,286