Business Segments - Business segments (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Segment Reporting Information [Line Items]
Revenues	$ 5,883	$ 5,742	$ 17,960	$ 16,370
Operating profit	1,358	1,668	4,820	1,309
Interest expense	129	768	989	2,418
Depreciation, depletion and amortization	1,431	1,821	4,390	6,350
Cash items	75,980		75,980		$ 22,749
Total identifiable net assets	536,297		536,297		505,488
Investments available for sale	115,308		115,308		165,212
Unallocated
Segment Reporting Information [Line Items]
Corporate expenses	0	0	0	(1,208)
Discontinued Operations, Disposed of by Sale [Member]
Segment Reporting Information [Line Items]
Operating profit	12	(307)	151	2,967
Interest expense	0	0	0	587
Depreciation, depletion and amortization	(24)	29	17	3,131
Total identifiable net assets	32		32		3,224
Asset Management
Segment Reporting Information [Line Items]
Revenues	430	568	1,733	1,717
Operating profit before corporate expenses	8	276	233	783
Corporate expenses	(168)	(34)	(470)	(146)
Capital expenditures	824	17	8,642	184
Depreciation, depletion and amortization	154	145	527	405
Total identifiable net assets	17,823		17,823		10,593
Mining royalty lands
Segment Reporting Information [Line Items]
Revenues	2,302	2,125	7,164	5,952
Operating profit before corporate expenses	2,103	1,961	6,605	5,497
Corporate expenses	(44)	(28)	(123)	(157)
Capital expenditures	0	0	0	0
Depreciation, depletion and amortization	36	55	130	145
Total identifiable net assets	38,734		38,734		37,991
Development
Segment Reporting Information [Line Items]
Revenues	307	330	892	944
Operating profit before corporate expenses	(629)	(139)	(1,747)	(1,146)
Corporate expenses	(479)	(408)	(1,219)	(1,110)
Capital expenditures	167	4,268	415	5,578
Depreciation, depletion and amortization	54	57	161	171
Total identifiable net assets	118,209		118,209		119,029
Stabilized Joint Venture
Segment Reporting Information [Line Items]
Revenues	2,844	2,719	8,171	7,757
Operating profit before corporate expenses	608	92	1,657	(915)
Corporate expenses	(41)	(52)	(116)	(289)
Capital expenditures	194	25	304	(33)
Depreciation, depletion and amortization	1,187	1,564	3,572	5,629
Total identifiable net assets	135,232		135,232		138,206
Corporate
Segment Reporting Information [Line Items]
Total identifiable net assets	34,979		34,979		$ 8,484
Total Segments
Segment Reporting Information [Line Items]
Revenues	5,883	5,742	17,960	16,370
Operating profit before corporate expenses	2,090	2,190	6,748	4,219
Operating profit	1,358	1,668	4,820	1,309
Corporate expenses	(732)	(522)	(1,928)	(2,910)
Interest expense	129	768	989	2,418
Capital expenditures	1,185	4,310	9,361	5,729
Depreciation, depletion and amortization	$ 1,431	$ 1,821	$ 4,390	$ 6,350