Summary of Significant Accounting Policies - Reconciliation of Activity Related to Allowance for Sales Returns (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Movement in Valuation Allowances and Reserves [Roll Forward]
Beginning balance	$ 8,148	$ 8,944	$ 7,334
Provision	38,444	35,746	36,980
Sales returns	(37,251)	(36,542)	(35,370)
Ending balance	$ 9,341	$ 8,148	$ 8,944