Notes Receivable Notes Receivable (Summary of Mezzanine Loans) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
Notes receivable	$ 93,478		$ 93,478		$ 83,058
Interest income	2,375	$ 1,658	4,607	$ 3,056
Mezzanine Loan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Notes receivable	92,548		92,548		82,634
Maximum loan commitment	115,172		115,172
Interest income	2,363	1,656	4,585	3,053
Mezzanine Loan | 1405 Point
Accounts, Notes, Loans and Financing Receivable [Line Items]
Notes receivable	25,633		25,633		22,444
Maximum loan commitment	$ 28,232		$ 28,232
Interest rate	8.00%		8.00%
Interest income	$ 483	429	$ 936	845
Mezzanine Loan | The Residences at Annapolis Junction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Notes receivable	45,230		45,230		43,021
Maximum loan commitment	$ 48,105		$ 48,105
Interest rate	10.00%		10.00%
Interest income	$ 1,124	1,016	$ 2,209	1,997
Mezzanine Loan | North Decatur Square
Accounts, Notes, Loans and Financing Receivable [Line Items]
Notes receivable	15,134		15,134		11,790
Maximum loan commitment	$ 25,712		$ 25,712
Interest rate	15.00%		15.00%
Interest income	$ 531	211	$ 992	211
Mezzanine Loan | Delray Plaza
Accounts, Notes, Loans and Financing Receivable [Line Items]
Notes receivable	6,551		6,551		$ 5,379
Maximum loan commitment	$ 13,123		$ 13,123
Interest rate	15.00%		15.00%
Interest income	$ 225	$ 0	$ 448	$ 0