Business Segments - Business segments (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Segment Reporting Information [Line Items]
Revenues	$ 6,098	$ 5,883	$ 17,730	$ 17,960
Operating profit	1,581	1,358	3,579	4,820
Corporate expenses	637	732	2,850	1,928
Interest expense	46	129	142	989
Depreciation, depletion and amortization	1,438	1,431	4,406	4,390
Cash items	61,548		61,548		$ 26,793
Total identifiable net assets	536,130		536,130		538,148
Investments available for sale	104,624		104,624		137,867
Asset Management
Segment Reporting Information [Line Items]
Revenues	721	430	2,089	1,733
Operating profit before corporate expenses	200	8	700	233
Corporate expenses	(165)	(168)	(738)	(470)
Capital expenditures	233	824	787	8,642
Depreciation, depletion and amortization	137	154	529	527
Total identifiable net assets	11,323		11,323		18,468
Mining royalty lands
Segment Reporting Information [Line Items]
Revenues	2,507	2,302	7,094	7,164
Operating profit before corporate expenses	2,291	2,103	6,486	6,605
Corporate expenses	(53)	(44)	(234)	(123)
Capital expenditures	0	0	0	0
Depreciation, depletion and amortization	60	36	160	130
Total identifiable net assets	37,617		37,617		38,409
Development
Segment Reporting Information [Line Items]
Revenues	290	307	862	892
Operating profit before corporate expenses	(659)	(629)	(2,136)	(1,747)
Corporate expenses	(381)	(479)	(1,710)	(1,219)
Capital expenditures	1,754	167	2,371	415
Depreciation, depletion and amortization	53	54	160	161
Total identifiable net assets	182,567		182,567		179,357
Stabilized Joint Venture
Segment Reporting Information [Line Items]
Revenues	2,580	2,844	7,685	8,171
Operating profit before corporate expenses	386	608	1,379	1,657
Corporate expenses	(38)	(41)	(168)	(116)
Capital expenditures	46	194	42	304
Depreciation, depletion and amortization	1,188	1,187	3,557	3,572
Total identifiable net assets	136,679		136,679		133,956
Total Segments
Segment Reporting Information [Line Items]
Revenues	6,098	5,883	17,730	17,960
Operating profit before corporate expenses	2,218	2,090	6,429	6,748
Operating profit	1,581	1,358	3,579	4,820
Corporate expenses	(637)	(732)	(2,850)	(1,928)
Interest expense	46	129	142	989
Capital expenditures	2,033	1,185	3,200	9,361
Depreciation, depletion and amortization	1,438	$ 1,431	4,406	$ 4,390
Corporate
Segment Reporting Information [Line Items]
Total identifiable net assets	$ 1,772		$ 1,772		$ 3,298