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Summary of Significant Accounting Policies - Summary of Adjustments in Allowances for Doubtful Accounts and Sales Adjustments (Detail) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Allowance for Doubtful Accounts Receivable [Roll Forward]      
Balance at beginning of year $ 3,396 $ 1,640 $ 1,725
Provisions for losses - accounts receivable 4,110 4,484 1,756
Collection of accounts previously written off 0 0 153
Deductions for accounts charged off or credits issued (2,079) (2,728) (1,994)
Balance at end of year $ 5,427 $ 3,396 $ 1,640