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Debt (Details) - USD ($)
6 Months Ended 12 Months Ended
May 04, 2017
Jun. 30, 2019
Dec. 31, 2018
Line of Credit Facility [Line Items]      
Revolving credit facility, maximum borrowing capacity $ 25,000,000.0    
Arrears paid on average unused portion 0.25%    
Outstanding balance of credit facility, amount requiring covenant compliance $ 5,000,000.0    
Borrowings outstanding   $ 0 $ 0
Line of Credit | LIBOR      
Line of Credit Facility [Line Items]      
Basis spread on variable rate 2.00%    
Line of Credit | Prime Rate      
Line of Credit Facility [Line Items]      
Basis spread on variable rate 1.00%