Business Segments - Business segments (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Segment Reporting Information [Line Items]
Revenues	$ 5,714	$ 5,075
Operating profit	1,355	97
Interest expense	588	843
Depreciation, depletion and amortization	1,487	2,398
Cash items	29,726		$ 22,749
Total identifiable net assets	506,123		505,488
Investments available for sale	148,778		165,212
Unallocated
Segment Reporting Information [Line Items]
Corporate expenses	0	(38)
Discontinued Operations, Disposed of by Sale [Member]
Segment Reporting Information [Line Items]
Operating profit	94	2,761
Interest expense	0	400
Depreciation, depletion and amortization	29	1,885
Total identifiable net assets	3,091		3,224
Asset Management
Segment Reporting Information [Line Items]
Revenues	641	581
Operating profit before corporate expenses	97	259
Corporate expenses	(163)	(3)
Capital expenditures	6,466	162
Depreciation, depletion and amortization	177	131
Total identifiable net assets	17,062		10,593
Mining royalty lands
Segment Reporting Information [Line Items]
Revenues	2,229	1,772
Operating profit before corporate expenses	2,044	1,618
Corporate expenses	(43)	(77)
Capital expenditures	0	0
Depreciation, depletion and amortization	52	54
Total identifiable net assets	38,021		37,991
Development
Segment Reporting Information [Line Items]
Revenues	269	297
Operating profit before corporate expenses	(553)	(387)
Corporate expenses	(399)	(419)
Capital expenditures	370	279
Depreciation, depletion and amortization	58	57
Total identifiable net assets	121,315		119,029
Stabilized Joint Venture
Segment Reporting Information [Line Items]
Revenues	2,575	2,425
Operating profit before corporate expenses	412	(714)
Corporate expenses	(40)	(142)
Capital expenditures	(117)	(243)
Depreciation, depletion and amortization	1,200	2,156
Total identifiable net assets	136,959		$ 138,206
Corporate
Segment Reporting Information [Line Items]
Total identifiable net assets	11,071	8,484
Total Segments
Segment Reporting Information [Line Items]
Revenues	5,714	5,075
Operating profit before corporate expenses	2,000	776
Operating profit	1,355	97
Corporate expenses	(645)	(679)
Interest expense	588	843
Capital expenditures	6,719	198
Depreciation, depletion and amortization	$ 1,487	$ 2,398