Balance Sheet Components - Additional Information (Detail) - USD ($)	3 Months Ended
	Jun. 30, 2018	Dec. 31, 2017
Inventory [Line Items]
Prepaid manufacturing	$ 17,880,000	$ 2,333,000
Cost of Sales
Inventory [Line Items]
Reserve for obsolescence inventories	600,000
Prepaid And Other Long-term Assets
Inventory [Line Items]
Prepayment for purchase of inventory	$ 0	$ 9,600,000