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Revenue Recognition - Allowance for Doubtful Accounts (Details) - USD ($)
$ in Thousands
3 Months Ended 9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Allowance for Doubtful Accounts Receivable [Roll Forward]        
Beginning balance $ 3,675 $ 3,607 $ 4,447 $ 5,728
Provision for credit losses 2,270 1,063 1,986 1,854
Write-off of uncollectible amounts, net of recoveries (307) (113) (795) (3,025)
Ending balance $ 5,638 $ 4,557 $ 5,638 $ 4,557
Warranty policy term     2 years