Additional Operations and Balance Sheet Information - Revenues, costs and expenses related to the Company's oil and gas operations (Details) - USD ($)	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Capitalized costs relating to oil and gas producing activities:
Unproved properties	$ 1,876,000	$ 1,896,000	$ 1,891,000
Proved properties	25,062,000	25,996,000	26,675,000
Total capitalized costs	26,938,000	27,892,000	28,566,000
Accumulated amortization	(24,848,000)	(24,454,000)	(24,015,000)
Total capitalized costs, net	$ 2,090,000	$ 3,438,000	$ 4,551,000