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Line of Credit (Details) - USD ($)
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Oct. 27, 2017
Sep. 19, 2016
Line of Credit Facility [Line Items]          
Credit line outstanding $ 138,000,000 $ 49,987,000      
Total interest expense on borrowings $ 2,700,000 $ 1,300,000 $ 300,000    
Line of Credit          
Line of Credit Facility [Line Items]          
Maximum borrowing capacity       $ 170,000,000.0 $ 125,000,000.0
Line of Credit | US Bank          
Line of Credit Facility [Line Items]          
Effective interest rate 3.04%        
Commitment fees $ 0        
Line of Credit | US Bank | London Interbank Offered Rate (LIBOR) | Minimum          
Line of Credit Facility [Line Items]          
Basis spread on variable rate 1.25%        
Line of Credit | US Bank | London Interbank Offered Rate (LIBOR) | Maximum          
Line of Credit Facility [Line Items]          
Basis spread on variable rate 1.75%        
Line of Credit | US Bank | Base rate | Minimum          
Line of Credit Facility [Line Items]          
Basis spread on variable rate 0.00%        
Line of Credit | US Bank | Base rate | Maximum          
Line of Credit Facility [Line Items]          
Basis spread on variable rate 0.50%        
Line of Credit | US Bank | Enson          
Line of Credit Facility [Line Items]          
Subsidiary ownership percentage securing facility 65.00%        
Letter of Credit          
Line of Credit Facility [Line Items]          
Letter of credit outstanding amount $ 0