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Line of Credit (Details) - USD ($)
3 Months Ended 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Dec. 31, 2017
Line of Credit Facility [Line Items]          
Amount outstanding under line of credit $ 111,000,000   $ 111,000,000   $ 138,000,000
Interest expense on borrowings $ 1,400,000 $ 600,000 $ 2,500,000 $ 1,100,000  
Line of Credit | US Bank          
Line of Credit Facility [Line Items]          
Effective interest rate 3.84%   3.84%    
Line of Credit | US Bank | LIBOR | Minimum          
Line of Credit Facility [Line Items]          
Basis spread on variable rate     1.25%    
Line of Credit | US Bank | 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]          
Ownership interest used to secure obligations 65.00%   65.00%    
Line of Credit | Second Amended Credit Agreement          
Line of Credit Facility [Line Items]          
Maximum borrowing capacity $ 170,000,000   $ 170,000,000    
Commitment fees     0    
Letter of Credit          
Line of Credit Facility [Line Items]          
Letter of credit outstanding amount $ 0   $ 0