Business Segments - Business segments (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017	Jun. 30, 2017
Segment Reporting Information [Line Items]
Revenues	$ 5,742	$ 5,035	$ 16,370	$ 10,389
Operating profit	1,668	(259)	1,309	918
Interest expense	768	783	2,418	783
Depreciation, depletion and amortization	1,821	2,804	6,350	3,303
Cash items	69,052		69,052		$ 4,524
Total identifiable net assets	507,259		507,259		418,734
Investments available for sale	191,288		191,288		0
Unallocated
Segment Reporting Information [Line Items]
Corporate expenses	0	(323)	(1,208)	(1,306)
Discontinued Operations, Disposed of by Sale [Member]
Segment Reporting Information [Line Items]
Revenues	219	7,019	11,876	20,347
Operating profit	(307)	3,087	2,967	9,300
Interest expense	0	468	587	1,087
Depreciation, depletion and amortization	29	1,965	3,131	5,727
Total identifiable net assets	3,194		3,194		176,694
Asset Management
Segment Reporting Information [Line Items]
Revenues	568	559	1,717	1,710
Operating profit (loss) before corporate expenses	276	249	783	771
Corporate expenses	(34)	(27)	(146)	(118)
Capital expenditures	17	131	184	162
Depreciation, depletion and amortization	145	125	405	385
Total identifiable net assets	10,687		10,687		2,960
Mining royalty lands
Segment Reporting Information [Line Items]
Revenues	2,125	1,786	5,952	5,381
Operating profit (loss) before corporate expenses	1,961	1,667	5,497	4,993
Corporate expenses	(28)	(30)	(157)	(124)
Capital expenditures	0	0	0	0
Depreciation, depletion and amortization	55	17	145	91
Total identifiable net assets	38,307		38,307		38,656
Land Development and Construction
Segment Reporting Information [Line Items]
Revenues	330	323	944	931
Operating profit (loss) before corporate expenses	(139)	(390)	(1,146)	(1,168)
Corporate expenses	(408)	(210)	(1,110)	(935)
Capital expenditures	4,268	292	5,578	1,999
Depreciation, depletion and amortization	57	98	171	263
Total identifiable net assets	51,801		51,801		46,684
RiverFront on the Anacostia
Segment Reporting Information [Line Items]
Revenues	2,719	2,367	7,757	2,367
Operating profit (loss) before corporate expenses	92	(1,168)	(915)	(1,168)
Corporate expenses	(52)	(27)	(289)	(27)
Capital expenditures	25	331	(33)	331
Depreciation, depletion and amortization	1,564	2,564	5,629	2,564
Total identifiable net assets	138,853		138,853		$ 144,386	$ 151,837
Corporate
Segment Reporting Information [Line Items]
Total identifiable net assets	4,077	4,830	4,077	4,830
Total Segments
Segment Reporting Information [Line Items]
Revenues	5,742	5,035	16,370	10,389
Operating profit (loss) before corporate expenses	2,190	358	4,219	3,428
Operating profit	1,668	(259)	1,309	918
Corporate expenses	(522)	(617)	(2,910)	(2,510)
Interest expense	768	783	2,418	783
Capital expenditures	4,310	754	5,729	2,492
Depreciation, depletion and amortization	$ 1,821	$ 2,804	$ 6,350	$ 3,303