Business Segments - Business segments (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Jun. 30, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting Information [Line Items]
Revenues	$ 12,054	$ 9,776	$ 30,736	$ 28,634
Operating profit	2,828	4,308	10,218	9,783
Interest expense	1,251	273	1,870	1,080
Depreciation, depletion and amortization	4,769	2,160	9,030	6,155
Cash items	2,630	0	2,630	0		$ 0	$ 0
Total identifiable net assets	416,092		416,092			266,560
Asset Management
Segment Reporting Information [Line Items]
Revenues	7,578	7,323	22,057	21,824
Operating profit before corporate expenses	3,336	3,245	10,071	9,986
Corporate expenses	(350)	(339)	(1,424)	(1,213)
Interest expense	374	273	993	1,080
Capital expenditures	1,273	10,276	6,061	11,510
Depreciation, depletion and amortization	2,090	2,071	6,112	5,891
Total identifiable net assets	180,827		180,827			169,736
Mining royalty lands
Segment Reporting Information [Line Items]
Revenues	1,786	2,037	5,381	5,874
Operating profit before corporate expenses	1,667	1,915	4,993	5,504
Corporate expenses	(30)	(49)	(124)	(176)
Capital expenditures	0	99	0	205
Depreciation, depletion and amortization	17	24	91	70
Total identifiable net assets	38,744		38,744			39,259
Land Development and Construction
Segment Reporting Information [Line Items]
Revenues	323	416	931	936
Operating profit before corporate expenses	(390)	(196)	(1,168)	(3,359)
Corporate expenses	(210)	(268)	(935)	(959)
Capital expenditures	2,852	4,210	6,203	5,300
Depreciation, depletion and amortization	98	65	263	194
Total identifiable net assets	44,162		44,162			57,126
RiverFront on the Anacostia
Segment Reporting Information [Line Items]
Revenues	2,367	0	2,367	0
Operating profit before corporate expenses	(1,168)	0	(1,168)	0
Corporate expenses	(27)	0	(27)	0
Interest expense	877	0	877	0
Capital expenditures	331	0	331	0
Depreciation, depletion and amortization	2,564	0	2,564	0
Cash items					$ 2,295
Total identifiable net assets	146,718		146,718		$ 151,837	0
Corporate
Segment Reporting Information [Line Items]
Total identifiable net assets	3,011		3,011			$ 439
Total Segments
Segment Reporting Information [Line Items]
Revenues	12,054	9,776	30,736	28,634
Operating profit	2,828	4,308	10,218	9,783
Corporate expenses	(617)	(656)	(2,510)	(2,348)
Interest expense	1,251	273	1,870	1,080
Capital expenditures	4,456	14,585	12,595	17,015
Depreciation, depletion and amortization	$ 4,769	$ 2,160	$ 9,030	$ 6,155