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Debt (Schedule of Lines of Credit Outstanding) (Details) - USD ($)
$ in Thousands
Mar. 31, 2018
Dec. 31, 2017
Dec. 15, 2015
Line of Credit Facility [Line Items]      
Total lines of credit $ 135,000 $ 107,000  
Johns Creek Carrollton Allen | Inwood National Bank      
Line of Credit Facility [Line Items]      
Total lines of credit 45,000 32,000  
Unsecured Debt | Unsecured revolving credit facility | Citibank, N.A. and Credit Suisse AG, Cayman Islands Branch      
Line of Credit Facility [Line Items]      
Total lines of credit $ 90,000 $ 75,000 $ 40,000