Stockholders' Equity (Compensation Cost of Warrant and Option Issuances Recognized) (Details) - Warrants and Options [Member] - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Unproved Oil and Gas Properties [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation cost of warrant and option issuances	$ 348,000	$ 256,000	$ 213,000
Non Employees [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation cost of warrant and option issuances	302,000	276,000	360,000
Employees and Directors [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation cost of warrant and option issuances	$ 1,553,000	$ 2,448,000	$ 2,726,000