Notes Receivable and Current Expected Credit Losses (Summary of Mezzanine Loans) (Details) - USD ($)	Sep. 30, 2020	Sep. 22, 2020	May 31, 2020	Mar. 03, 2020	Dec. 31, 2019
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing receivable guarantee premium	$ 3,034,000				$ 5,271,000
Allowance for credit losses	(3,052,000)				0
Total notes receivable	168,716,000				159,371,000
Mezzanine Loan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Notes receivable	168,720,000				152,953,000
Maximum loan commitment	226,205,000
Total notes receivable	168,702,000
Mezzanine Loan | The Residences at Annapolis Junction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Notes receivable	42,767,000				40,049,000
Maximum loan commitment	$ 48,105,000
Interest rate	10.00%
Mezzanine Loan | Delray Plaza
Accounts, Notes, Loans and Financing Receivable [Line Items]
Notes receivable	$ 15,493,000				12,995,000
Maximum loan commitment	$ 17,000,000			$ 17,000,000.0
Interest rate	15.00%
Additional funds available	$ 2,000,000.0			$ 2,000,000.0
Interest rate	6.00%			6.00%
Mezzanine Loan | Nexton Square
Accounts, Notes, Loans and Financing Receivable [Line Items]
Notes receivable	$ 0	$ 16,400,000			15,097,000
Maximum loan commitment	$ 17,000,000
Interest rate	10.00%
Mezzanine Loan | Interlock Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Notes receivable	$ 82,351,000				59,224,000
Maximum loan commitment	$ 103,000,000
Interest rate	15.00%
Additional funds available			$ 8,000,000.0
Mezzanine Loan | Solis Apartments at Interlock
Accounts, Notes, Loans and Financing Receivable [Line Items]
Notes receivable	$ 28,109,000				25,588,000
Maximum loan commitment	$ 41,100,000
Interest rate	13.00%
Other Notes Receivable
Accounts, Notes, Loans and Financing Receivable [Line Items]
Notes receivable	$ 14,000				$ 1,147,000