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Line of Credit (Details) - USD ($)
 12 Months Ended
Jun. 13, 2016
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Line of Credit Facility [Line Items]        
Letters of credit outstanding amount   $ 4,630,000    
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   295,370,000    
Credit facility commitment fees   $ 609,000 $ 630,000 $ 645,000
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%