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Debt - Senior Unsecured Credit Facility (Details) - USD ($)
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Line of Credit Facility [Line Items]      
Increase in credit facility $ 600,000,000    
Percent of unused portion, line of credit facility, triggering lower commitment fee percentage 50.00%    
Senior unsecured credit facility $ 0    
Unsecured term loan | LIBOR      
Line of Credit Facility [Line Items]      
Interest and unused credit facility fees 1,000,000 $ 1,300,000 $ 1,100,000
Senior unsecured credit facility      
Line of Credit Facility [Line Items]      
Maximum borrowing capacity 300,000,000    
Senior unsecured credit facility $ 0 $ 0  
Senior unsecured credit facility | LIBOR      
Line of Credit Facility [Line Items]      
Basis spread on variable rate 1.50%    
Minimum      
Line of Credit Facility [Line Items]      
Commitment fee on unused percentage 0.20%    
Maximum      
Line of Credit Facility [Line Items]      
Commitment fee on unused percentage 0.30%