Notes Receivable Notes Receivable (Summary of Mezzanine Loans) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Sep. 18, 2018	Dec. 31, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
Notes receivable	$ 100,486		$ 100,486			$ 83,058
Interest income	2,545	$ 1,910	7,152	$ 4,966
Mezzanine Loan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Notes receivable	99,230		99,230			82,634
Maximum loan commitment	121,447		121,447
Interest income	2,529	1,909	7,113	4,962
Mezzanine Loan | 1405 Point
Accounts, Notes, Loans and Financing Receivable [Line Items]
Notes receivable	28,133		28,133			22,444
Maximum loan commitment	$ 28,232		$ 28,232
Interest rate	8.00%		8.00%
Interest income	$ 547	443	$ 1,483	1,288
Mezzanine Loan | The Residences at Annapolis Junction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Notes receivable	46,396		46,396			43,021
Maximum loan commitment	$ 48,105		$ 48,105
Interest rate	10.00%		10.00%
Interest income	$ 1,166	1,054	$ 3,374	3,051
Mezzanine Loan | North Decatur Square
Accounts, Notes, Loans and Financing Receivable [Line Items]
Notes receivable	15,703		15,703			11,790
Maximum loan commitment	$ 29,673		$ 29,673		$ 29,700
Interest rate	15.00%		15.00%
Interest income	$ 569	412	$ 1,561	623
Mezzanine Loan | Delray Plaza
Accounts, Notes, Loans and Financing Receivable [Line Items]
Notes receivable	6,779		6,779			5,379
Maximum loan commitment	$ 13,123		$ 13,123
Interest rate	15.00%		15.00%
Interest income	$ 228	0	$ 676	0
Mezzanine Loan | Nexton Square
Accounts, Notes, Loans and Financing Receivable [Line Items]
Notes receivable	2,219		2,219			$ 0
Maximum loan commitment	$ 2,314		$ 2,314
Interest rate	10.00%		10.00%
Interest income	$ 19	$ 0	$ 19	$ 0