Business Segments - Business segments (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017	Jun. 30, 2017	Dec. 31, 2016
Segment Reporting Information [Line Items]
Revenues	$ 12,622	$ 9,322
Operating profit	2,858	3,404
Interest expense	1,243	248
Depreciation, depletion and amortization	4,283	2,059
Cash items	6,961	106	$ 4,524		$ 0
Total identifiable net assets	417,984		418,734
Asset Management
Segment Reporting Information [Line Items]
Revenues	8,128	7,285
Operating profit (loss) before corporate expenses	3,767	3,501
Corporate expenses	(788)	(753)
Interest expense	290	248
Capital expenditures	902	2,259
Depreciation, depletion and amortization	2,016	1,965
Total identifiable net assets	186,711		179,654
Mining royalty lands
Segment Reporting Information [Line Items]
Revenues	1,772	1,762
Operating profit (loss) before corporate expenses	1,618	1,625
Corporate expenses	(77)	(66)
Capital expenditures	0	0
Depreciation, depletion and amortization	54	39
Total identifiable net assets	38,579		38,656
Land Development and Construction
Segment Reporting Information [Line Items]
Revenues	297	275
Operating profit (loss) before corporate expenses	(387)	(395)
Corporate expenses	(419)	(508)
Capital expenditures	925	1,470
Depreciation, depletion and amortization	57	55
Total identifiable net assets	39,220		46,684
RiverFront on the Anacostia
Segment Reporting Information [Line Items]
Revenues	2,425	0
Operating profit (loss) before corporate expenses	(714)	0
Corporate expenses	(142)	0
Interest expense	953	0
Capital expenditures	(243)	0
Depreciation, depletion and amortization	2,156	0
Cash items				$ 2,295
Total identifiable net assets	142,121		$ 144,386	$ 151,837
Corporate
Segment Reporting Information [Line Items]
Total identifiable net assets	4,392	4,830
Total Segments
Segment Reporting Information [Line Items]
Revenues	12,622	9,322
Operating profit	2,858	3,404
Corporate expenses	(1,426)	(1,327)
Interest expense	1,243	248
Capital expenditures	1,584	3,729
Depreciation, depletion and amortization	$ 4,283	$ 2,059