Business Segments - Business segments (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
Segment Reporting Information [Line Items]
Revenues	$ 5,783	$ 5,714
Operating profit	794	1,355
Corporate expenses	1,187	645
Interest expense	51	588
Depreciation, depletion and amortization	1,468	1,487
Cash items	11,564		$ 26,793
Total identifiable net assets	534,699		538,148
Investments available for sale	148,667		137,867
Asset Management
Segment Reporting Information [Line Items]
Revenues	652	641
Operating profit before corporate expenses	177	97
Corporate expenses	(308)	(163)
Capital expenditures	213	6,466
Depreciation, depletion and amortization	192	177
Total identifiable net assets	18,644		18,468
Mining royalty lands
Segment Reporting Information [Line Items]
Revenues	2,185	2,229
Operating profit before corporate expenses	2,001	2,044
Corporate expenses	(97)	(43)
Capital expenditures	0	0
Depreciation, depletion and amortization	38	52
Total identifiable net assets	38,494		38,409
Development
Segment Reporting Information [Line Items]
Revenues	293	269
Operating profit before corporate expenses	(774)	(553)
Corporate expenses	(712)	(399)
Capital expenditures	297	370
Depreciation, depletion and amortization	54	58
Total identifiable net assets	176,575		179,357
Stabilized Joint Venture
Segment Reporting Information [Line Items]
Revenues	2,653	2,575
Operating profit before corporate expenses	577	412
Corporate expenses	(70)	(40)
Capital expenditures	(23)	(117)
Depreciation, depletion and amortization	1,184	1,200
Total identifiable net assets	132,801		133,956
Corporate
Segment Reporting Information [Line Items]
Total identifiable net assets	7,954		$ 3,298
Total Segments
Segment Reporting Information [Line Items]
Revenues	5,783	5,714
Operating profit before corporate expenses	1,981	2,000
Operating profit	794	1,355
Corporate expenses	(1,187)	(645)
Interest expense	51	588
Capital expenditures	487	6,719
Depreciation, depletion and amortization	$ 1,468	$ 1,487