The Company and Significant Accounting Policies - Summary of Amortized Cost and Unrealized (Gains) Losses of Available for Sale Securities (Detail) $ in Thousands	6 Months Ended
	Jun. 30, 2018 USD ($)
Certificates of Deposit
Amortized Cost
Due after one year through five years	$ 8,282
Total	8,282
Unrealized (gains) losses
Due after one year through five years	(64)
Total	(64)
Corporate Bonds
Amortized Cost
Due within a year	10,012
Due after one year through five years	115,797
Total	125,809
Unrealized (gains) losses
Due within a year	(33)
Due after one year through five years	(1,558)
Total	$ (1,591)