Debt - Revolving Credit Facilities - Narrative (Details) - USD ($)
6 Months Ended
Jun. 30, 2018
Dec. 31, 2017
Aug. 18, 2016
Unsecured Debt      
Line of Credit Facility [Line Items]      
Line of credit facility, accordion feature, increase limit $ 1,000,000,000    
Commitment fee 0.25%    
Origination costs $ 4,800,000   $ 3,400,000
Line of credit facility, remaining borrowing capacity $ 453,500,000    
Unsecured Debt | Minimum      
Line of Credit Facility [Line Items]      
Commitment fee 0.125%    
Unsecured Debt | Maximum      
Line of Credit Facility [Line Items]      
Commitment fee 0.30%    
Unsecured Debt | LIBOR      
Line of Credit Facility [Line Items]      
Basis spread on variable rate 1.25%    
Unsecured Debt | LIBOR | Minimum      
Line of Credit Facility [Line Items]      
Basis spread on variable rate 0.875%    
Unsecured Debt | LIBOR | Maximum      
Line of Credit Facility [Line Items]      
Basis spread on variable rate 1.55%    
Revolving Credit Facility      
Line of Credit Facility [Line Items]      
Total debt, gross $ 346,500,000    
Revolving Credit Facility | Deferred Costs And Other Assets      
Line of Credit Facility [Line Items]      
Unamortized deferred financing costs $ 1,000,000 $ 1,600,000