Business Segments - Business segments (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
Segment Reporting Information [Line Items]
Revenues	$ 5,849	$ 6,363	$ 11,632	$ 12,077
Operating profit	1,204	2,107	1,998	3,462
Corporate expenses	1,026	551	2,213	1,196
Interest expense	45	272	96	860
Depreciation, depletion and amortization	1,500	1,472	2,968	2,959
Cash items	34,481		34,481		$ 26,793
Total identifiable net assets	534,117		534,117		538,148
Investments available for sale	130,058		130,058		137,867
Asset Management
Segment Reporting Information [Line Items]
Revenues	716	662	1,368	1,303
Operating profit before corporate expenses	323	128	500	225
Corporate expenses	(265)	(139)	(573)	(302)
Capital expenditures	341	1,352	554	7,818
Depreciation, depletion and amortization	200	196	392	373
Total identifiable net assets	18,813		18,813		18,468
Mining royalty lands
Segment Reporting Information [Line Items]
Revenues	2,402	2,633	4,587	4,862
Operating profit before corporate expenses	2,194	2,458	4,195	4,502
Corporate expenses	(84)	(36)	(181)	(79)
Capital expenditures	0	0	0	0
Depreciation, depletion and amortization	62	42	100	94
Total identifiable net assets	37,911		37,911		38,409
Development
Segment Reporting Information [Line Items]
Revenues	279	316	572	585
Operating profit before corporate expenses	(703)	(565)	(1,477)	(1,118)
Corporate expenses	(617)	(341)	(1,329)	(740)
Capital expenditures	320	(122)	617	248
Depreciation, depletion and amortization	53	49	107	107
Total identifiable net assets	173,334		173,334		179,357
Stabilized Joint Venture
Segment Reporting Information [Line Items]
Revenues	2,452	2,752	5,105	5,327
Operating profit before corporate expenses	416	637	993	1,049
Corporate expenses	(60)	(35)	(130)	(75)
Capital expenditures	19	227	(4)	110
Depreciation, depletion and amortization	1,185	1,185	2,369	2,385
Total identifiable net assets	131,652		131,652		133,956
Corporate
Segment Reporting Information [Line Items]
Total identifiable net assets	7,868		7,868		$ 3,298
Total Segments
Segment Reporting Information [Line Items]
Revenues	5,849	6,363	11,632	12,077
Operating profit before corporate expenses	2,230	2,658	4,211	4,658
Operating profit	1,204	2,107	1,998	3,462
Corporate expenses	(1,026)	(551)	(2,213)	(1,196)
Interest expense	45	272	96	860
Capital expenditures	680	1,457	1,167	8,176
Depreciation, depletion and amortization	$ 1,500	$ 1,472	$ 2,968	$ 2,959