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Revenue Recognition - Allowance for Doubtful Accounts (Details) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended
Jun. 30, 2019
Jun. 30, 2018
Jun. 30, 2019
Jun. 30, 2018
Allowance for Doubtful Accounts Receivable [Roll Forward]        
Beginning balance $ 5,474 $ 3,809 $ 5,610 $ 4,447
Provision for credit losses 265 311 142 (284)
Write-off of uncollectible amounts, net of recoveries (240) (445) (253) (488)
Ending balance $ 5,499 $ 3,675 $ 5,499 $ 3,675
Warranty policy term     2 years