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Summary of Significant Accounting Policies - Allowance for Doubtful Accounts (Detail) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Allowance for Doubtful Accounts Receivable [Roll Forward]      
Beginning Balance $ 405 $ 1,958 $ 1,263
Provision for Credit Losses 685 906 1,494
Charge-offs (531) (2,459) (799)
Ending Balance $ 559 $ 405 $ 1,958