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Debt - Senior Unsecured Credit Facility (Details)
12 Months Ended
Dec. 31, 2018
USD ($)
Dec. 31, 2017
USD ($)
Dec. 31, 2016
USD ($)
Feb. 28, 2019
USD ($)
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 1,200,000 $ 1,000,000 $ 1,300,000  
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%      
Line Of Credit Facility Covenant Actual Results        
Line of Credit Facility [Line Items]        
Maximum leverage ratio 0.275      
Subsequent Event        
Line of Credit Facility [Line Items]        
Senior unsecured credit facility       $ 45,000,000