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Product Warranty (Details) - USD ($)
$ in Thousands
6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Dec. 31, 2017
Product Warranties Disclosures [Abstract]      
Product Warranty Accrual $ 2,039   $ 2,074
Summary of product warranty activity      
Beginning balance 2,074 $ 2,280  
Charges to expense 1,156 500  
Costs incurred (1,191) (373)  
Ending balance $ 2,039 $ 2,407