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Debt (Narrative) (Details) - USD ($)
Mar. 31, 2019
Dec. 31, 2018
Dec. 15, 2015
Jul. 30, 2015
Debt Instrument [Line Items]        
Costs associated with amendment $ 1,478,000 $ 1,614,000    
Borrowings on lines of credit 206,522,000 $ 200,386,000    
Inwood National Bank        
Debt Instrument [Line Items]        
Line of Credit Facility, Maximum Borrowing Capacity $ 75,000,000      
Inwood National Bank | Johns Creek Carrollton Allen        
Debt Instrument [Line Items]        
Line of Credit Facility, Maximum Borrowing Capacity       $ 50,000,000.0
Line of Credit Facility, Interest Rate at Period End 5.25%      
Citibank, N.A. and Credit Suisse AG, Cayman Islands Branch | Unsecured Revolving Credit Facility [Member] | Unsecured Debt [Member]        
Debt Instrument [Line Items]        
Borrowings on lines of credit     $ 40,000,000  
Citibank, N.A. and Credit Suisse AG, Cayman Islands Branch | Unsecured Revolving Credit Facility [Member] | Unsecured Debt [Member] | Minimum [Member]        
Debt Instrument [Line Items]        
Line of Credit Facility, Interest Rate at Period End 4.98%      
Citibank, N.A. and Credit Suisse AG, Cayman Islands Branch | Unsecured Revolving Credit Facility [Member] | Unsecured Debt [Member] | Maximum [Member]        
Debt Instrument [Line Items]        
Line of Credit Facility, Interest Rate at Period End 4.99%