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Line of Credit (Details) - USD ($)
1 Months Ended 12 Months Ended
Jun. 13, 2016
Jul. 31, 2017
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Line of Credit Facility [Line Items]          
Credit facility principal payment     $ 30,000,000 $ 10,000,000 $ 0
Letters of credit outstanding amount     13,813,000    
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 borrowings   $ 40,000,000      
Credit facility principal payment     30,000,000 10,000,000  
Credit facility remaining borrowing capacity     286,187,000    
Credit facility commitment fees     645,000 $ 781,000 $ 531,000
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 [Member]          
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%        
Weighted Average | Revolving Credit Facility          
Line of Credit Facility [Line Items]          
Credit facility interest rate during the period     2.93%