Balance Sheet Components - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2018	Dec. 31, 2017
Inventory [Line Items]
Prepaid manufacturing	$ 4,071	$ 4,071	$ 2,333
Reserve for obsolescence inventories	2,200	2,200
Cost of Sales
Inventory [Line Items]
Reserve for obsolescence inventories	1,700	2,200
Prepaid And Other Long-term Assets
Inventory [Line Items]
Long-term prepaid manufacturing	$ 12,500	$ 12,500	$ 9,600