Debt (Schedule of Lines of Credit Outstanding) (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Line of Credit Facility [Line Items]
Debt issuance costs, net of amortization	$ (1,035)	$ (1,227)
Total lines of credit	198,965	105,773
Johns Creek Carrollton Allen | Inwood National Bank
Line of Credit Facility [Line Items]
Lines of credit, gross	57,500	32,000
Unsecured revolving credit facility | Unsecured Debt | Citibank, N.A., Credit Suisse AG, Cayman Islands Branch, Flagstar Bank and JP Morgan Chase
Line of Credit Facility [Line Items]
Lines of credit, gross	$ 142,500	$ 75,000