Investments - Fair Values and Gross Unrealized Losses of Available-for-Sale Securities Aggregated by Investment Category (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Continuous loss position, less than 12 months, fair value	$ 280,570	$ 727,361
Continuous loss position, less than 12 months, gross unrealized losses	(204)	(2,608)
Continuous loss position, 12 months or greater, fair value	16,264	356,413
Continuous loss position, 12 months or greater, gross unrealized losses	(15)	(1,968)
Continuous loss position, total, fair value	296,834	1,083,774
Continuous loss position, total, gross unrealized losses	(219)	(4,576)
Commercial paper
Debt Securities, Available-for-sale [Line Items]
Continuous loss position, less than 12 months, fair value	20,752
Continuous loss position, less than 12 months, gross unrealized losses	(9)
Continuous loss position, 12 months or greater, fair value	0
Continuous loss position, 12 months or greater, gross unrealized losses	0
Continuous loss position, total, fair value	20,752
Continuous loss position, total, gross unrealized losses	(9)
Corporate notes and bonds
Debt Securities, Available-for-sale [Line Items]
Continuous loss position, less than 12 months, fair value	242,012	714,605
Continuous loss position, less than 12 months, gross unrealized losses	(181)	(2,603)
Continuous loss position, 12 months or greater, fair value	16,264	294,956
Continuous loss position, 12 months or greater, gross unrealized losses	(15)	(1,615)
Continuous loss position, total, fair value	258,276	1,009,561
Continuous loss position, total, gross unrealized losses	(196)	(4,218)
Certificates of deposit
Debt Securities, Available-for-sale [Line Items]
Continuous loss position, less than 12 months, fair value		1,000
Continuous loss position, less than 12 months, gross unrealized losses		0
Continuous loss position, 12 months or greater, fair value		0
Continuous loss position, 12 months or greater, gross unrealized losses		0
Continuous loss position, total, fair value		1,000
Continuous loss position, total, gross unrealized losses		0
U.S. government and agency securities
Debt Securities, Available-for-sale [Line Items]
Continuous loss position, less than 12 months, fair value	17,806	11,756
Continuous loss position, less than 12 months, gross unrealized losses	(14)	(5)
Continuous loss position, 12 months or greater, fair value	0	61,457
Continuous loss position, 12 months or greater, gross unrealized losses	0	(353)
Continuous loss position, total, fair value	17,806	73,213
Continuous loss position, total, gross unrealized losses	$ (14)	$ (358)