Mortgages and Other Notes Payable - Schedule of Debt (Details) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2021	Dec. 31, 2020	Jun. 04, 2021	Mar. 04, 2021
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Mortgages and notes payable	$ 211,333	$ 242,711
Mortgages and other notes payable | 600 Las Colinas
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Mortgages and notes payable	$ 35,212	35,589
Effective Interest Rate	5.30%
Mortgages and other notes payable | 770 South Post Oak
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Mortgages and notes payable	$ 11,763	11,871
Effective Interest Rate	4.40%
Mortgages and other notes payable | Athens
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Mortgages and notes payable	$ 1,155	1,155
Effective Interest Rate	5.90%
Mortgages and other notes payable | Chelsea
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Mortgages and notes payable	$ 8,116	8,194
Effective Interest Rate	3.40%
Mortgages and other notes payable | EQK Portage - Land
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Mortgages and notes payable	$ 3,350	3,350
Effective Interest Rate	10.00%
Mortgages and other notes payable | HSW Partners
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Mortgages and notes payable	$ 0	17,790
Effective Interest Rate	9.50%
Mortgages and other notes payable | Forest Grove
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Mortgages and notes payable	$ 7,325	$ 7,333
Effective Interest Rate	3.75%
Mortgages and other notes payable | Forest Grove | Prime Rate
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Variable interest rate	0.50%	0.50%
Mortgages and other notes payable | Landing Bayou
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Mortgages and notes payable	$ 14,526	$ 14,643
Effective Interest Rate	3.50%
Mortgages and other notes payable | Legacy at Pleasant Grove
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Mortgages and notes payable	$ 13,504	13,653
Effective Interest Rate	3.60%
Mortgages and other notes payable | McKinney 36 Land
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Mortgages and notes payable	$ 732	820
Effective Interest Rate	8.00%
Mortgages and other notes payable | New Concept Energy
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Mortgages and notes payable	$ 3,542	3,542
Effective Interest Rate	6.00%
Mortgages and other notes payable | Overlook at Allenville Phase II
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Mortgages and notes payable	$ 0	15,621
Effective Interest Rate	3.80%
Mortgages and other notes payable | Parc at Denham Springs Phase II
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Mortgages and notes payable	$ 16,046	16,128
Effective Interest Rate	4.10%
Mortgages and other notes payable | RCM HC Enterprises
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Mortgages and notes payable	$ 5,086	0	$ 1,986
Effective Interest Rate	9.50%
Mortgages and other notes payable | Stanford Center
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Mortgages and notes payable	$ 39,063	39,093
Effective Interest Rate	6.00%
Mortgages and other notes payable | Sugar Mill Phase III
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Mortgages and notes payable	$ 9,257	9,298
Effective Interest Rate	4.50%
Mortgages and other notes payable | Toulon
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Mortgages and notes payable	$ 13,849	13,975
Effective Interest Rate	3.20%
Mortgages and other notes payable | Villas at Bon Secour
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Mortgages and notes payable	$ 10,149	10,280
Effective Interest Rate	4.00%
Mortgages and other notes payable | Vista Ridge
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Mortgages and notes payable	$ 9,905	9,979
Effective Interest Rate	4.00%
Mortgages and other notes payable | Windmill Farms
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Mortgages and notes payable	$ 8,753	$ 10,397
Effective Interest Rate	6.00%			5.00%