Summary of Investments (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 185,172	$ 221,347
Unrealized Gain	237	54
Unrealized (Loss)	(14)	(253)
Estimated Fair Value	185,395	221,148
Corporate Debt Securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	111,240	159,348
Unrealized Gain	167	5
Unrealized (Loss)	(7)	(238)
Estimated Fair Value	111,400	159,115
Government and Agency Securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	54,070	25,764
Unrealized Gain	32
Unrealized (Loss)	(7)	(14)
Estimated Fair Value	54,095	25,750
Commercial Paper
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	19,862	36,235
Unrealized Gain	38	49
Unrealized (Loss)		(1)
Estimated Fair Value	$ 19,900	$ 36,283