Business segments (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Segment Reporting Information [Line Items]
Revenues	$ 10,696	$ 9,628	$ 20,810	$ 18,335
Operating profit before corporate expenses	4,136	3,373	7,944	5,572
Corporate expenses	(1,369)	(1,307)	(2,323)	(2,142)
Operating profit	2,767	2,066	5,621	3,430
Interest expense	1,129	739	2,135	1,477
Depreciation, depletion and amortization			5,724	5,890
Capital expenditures	979	13,775	2,185	17,411
Asset Management [Member]
Segment Reporting Information [Line Items]
Revenues	1,420	912	2,490	1,751
Operating profit before corporate expenses	681	419	1,158	711
Corporate expenses	(271)	(225)	(453)	(369)
Depreciation, depletion and amortization	359	230	637	464
Capital expenditures	65	145	545	595
Mining Properties [Member]
Segment Reporting Information [Line Items]
Revenues	3,264	2,883	6,546	5,308
Operating profit before corporate expenses	2,886	2,498	5,783	4,681
Corporate expenses	(154)	(148)	(261)	(242)
Depreciation, depletion and amortization	151	189	334	244
Capital expenditures	0	11,126	0	11,217
Development [Member]
Segment Reporting Information [Line Items]
Revenues	467	408	953	791
Operating profit before corporate expenses	(472)	(581)	(933)	(1,299)
Corporate expenses	(815)	(816)	(1,389)	(1,337)
Depreciation, depletion and amortization	41	47	96	92
Capital expenditures	867	2,426	1,461	5,379
Stabilized Joint Venture [Member]
Segment Reporting Information [Line Items]
Revenues	5,545	5,425	10,821	10,485
Operating profit before corporate expenses	1,041	1,037	1,936	1,479
Corporate expenses	(129)	(118)	(220)	(194)
Depreciation, depletion and amortization	2,268	2,402	4,532	4,966
Capital expenditures	47	78	179	220
Total Segments [Member]
Segment Reporting Information [Line Items]
Depreciation, depletion and amortization	$ 2,819	$ 2,868	$ 5,599	$ 5,766