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Summary of Significant Accounting Policies - Reconciliation of Activity Related to Allowance for Sales Returns (Detail) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Movement in Valuation Allowances and Reserves [Roll Forward]      
Beginning balance $ 9,341 $ 8,148 $ 8,944
Provision 37,521 38,444 35,746
Sales returns (31,392) (37,251) (36,542)
Ending balance $ 15,470 $ 9,341 $ 8,148