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Line of Credit (Details) - USD ($)
 12 Months Ended
Jun. 13, 2016
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Line of Credit Facility [Line Items]        
Letters of credit outstanding amount   $ 11,553,000    
Revolving Credit Facility        
Line of Credit Facility [Line Items]        
Debt instrument, term 5 years      
Credit facility maximum borrowing capacity $ 300,000,000      
Credit facility stated percentage 0.00%      
Credit facility accordion feature, increase limit $ 100,000,000      
Credit facility remaining borrowing capacity   288,447,000    
Credit facility commitment fees   $ 630,000 $ 645,000 $ 781,000
Revolving Credit Facility | London Interbank Offered Rate (LIBOR)        
Line of Credit Facility [Line Items]        
Credit facility variable interest rate basis spread (as a percent) 1.00%      
Revolving Credit Facility | Federal Funds Rate        
Line of Credit Facility [Line Items]        
Credit facility variable interest rate basis spread (as a percent) 0.50%      
Letter of Credit        
Line of Credit Facility [Line Items]        
Credit facility maximum borrowing capacity $ 15,000,000      
Credit facility, commitment fee percentage 1.25%      
Credit facility, fronting fee percentage 0.175%      
Minimum | Revolving Credit Facility        
Line of Credit Facility [Line Items]        
Credit facility commitment fee per annum on daily unused portion (as a percent) 0.15%      
Minimum | Revolving Credit Facility | Base Rate        
Line of Credit Facility [Line Items]        
Credit facility variable interest rate basis spread (as a percent) 0.25%      
Minimum | Revolving Credit Facility | London Interbank Offered Rate (LIBOR)        
Line of Credit Facility [Line Items]        
Credit facility variable interest rate basis spread (as a percent) 1.25%      
Maximum | Revolving Credit Facility        
Line of Credit Facility [Line Items]        
Credit facility commitment fee per annum on daily unused portion (as a percent) 0.30%      
Maximum | Revolving Credit Facility | Base Rate        
Line of Credit Facility [Line Items]        
Credit facility variable interest rate basis spread (as a percent) 1.00%      
Maximum | Revolving Credit Facility | London Interbank Offered Rate (LIBOR)        
Line of Credit Facility [Line Items]        
Credit facility variable interest rate basis spread (as a percent) 2.00%