Operating results and certain other financial data for the Company’s business segments (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Segment Reporting Information [Line Items]
Revenues	$ 8,495	$ 5,849	$ 14,348	$ 11,632
Operating profit before corporate expenses	891	2,230	3,112	4,211
Corporate expenses	(1,050)	(1,026)	(1,829)	(2,213)
Operating profit	(159)	1,204	1,283	1,998
Interest expense	446	45	1,371	96
Depreciation, depletion and amortization			5,951	3,084
Capital expenditures	3,458	680	6,845	1,167
Asset Management [Member]
Segment Reporting Information [Line Items]
Revenues	588	716	1,300	1,368
Operating profit before corporate expenses	128	323	359	500
Corporate expenses	(288)	(265)	(502)	(573)
Depreciation, depletion and amortization	134	200	271	392
Capital expenditures	139	341	218	554
Mining Properties [Member]
Segment Reporting Information [Line Items]
Revenues	2,634	2,402	4,949	4,587
Operating profit before corporate expenses	2,400	2,194	4,494	4,195
Corporate expenses	(108)	(84)	(189)	(181)
Depreciation, depletion and amortization	58	62	123	100
Capital expenditures	0	0	0	0
Development [Member]
Segment Reporting Information [Line Items]
Revenues	451	279	768	572
Operating profit before corporate expenses	(411)	(703)	(797)	(1,477)
Corporate expenses	(522)	(617)	(941)	(1,329)
Depreciation, depletion and amortization	53	53	106	107
Capital expenditures	2,907	320	6,206	617
Stabilized Joint Venture [Member]
Segment Reporting Information [Line Items]
Revenues	4,822	2,452	7,331	5,105
Operating profit before corporate expenses	(1,226)	416	(944)	993
Corporate expenses	(132)	(60)	(197)	(130)
Depreciation, depletion and amortization	4,143	1,185	5,331	2,369
Capital expenditures	412	19	421	(4)
Total Segments [Member]
Segment Reporting Information [Line Items]
Depreciation, depletion and amortization	$ 4,388	$ 1,500	$ 5,831	$ 2,968