Investments - Fair Values and Gross Unrealized Losses of Available-for-Sale Securities Aggregated by Investment Category (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Continuous unrealized loss position, less than 12 months, fair value	$ 296,482	$ 727,361
Continuous unrealized loss position, less than 12 months, gross unrealized losses	(249)	(2,608)
Continuous unrealized loss position, 12 months or greater, fair value	346,376	356,413
Continuous unrealized loss position, 12 months or greater, gross unrealized loss	(817)	(1,968)
Continuous unrealized loss position, fair value	642,858	1,083,774
Continuous unrealized loss position, gross unrealized losses	(1,066)	(4,576)
Commercial paper
Debt Securities, Available-for-sale [Line Items]
Continuous unrealized loss position, less than 12 months, fair value	35,844
Continuous unrealized loss position, less than 12 months, gross unrealized losses	(6)
Continuous unrealized loss position, 12 months or greater, fair value	0
Continuous unrealized loss position, 12 months or greater, gross unrealized loss	0
Continuous unrealized loss position, fair value	35,844
Continuous unrealized loss position, gross unrealized losses	(6)
Corporate notes and bonds
Debt Securities, Available-for-sale [Line Items]
Continuous unrealized loss position, less than 12 months, fair value	255,199	714,605
Continuous unrealized loss position, less than 12 months, gross unrealized losses	(243)	(2,603)
Continuous unrealized loss position, 12 months or greater, fair value	296,220	294,956
Continuous unrealized loss position, 12 months or greater, gross unrealized loss	(660)	(1,615)
Continuous unrealized loss position, fair value	551,419	1,009,561
Continuous unrealized loss position, gross unrealized losses	(903)	(4,218)
Certificates of deposit
Debt Securities, Available-for-sale [Line Items]
Continuous unrealized loss position, less than 12 months, fair value	2,050	1,000
Continuous unrealized loss position, less than 12 months, gross unrealized losses	0	0
Continuous unrealized loss position, 12 months or greater, fair value	0	0
Continuous unrealized loss position, 12 months or greater, gross unrealized loss	0	0
Continuous unrealized loss position, fair value	2,050	1,000
Continuous unrealized loss position, gross unrealized losses	0	0
U.S. government and agency securities
Debt Securities, Available-for-sale [Line Items]
Continuous unrealized loss position, less than 12 months, fair value	3,389	11,756
Continuous unrealized loss position, less than 12 months, gross unrealized losses	0	(5)
Continuous unrealized loss position, 12 months or greater, fair value	50,156	61,457
Continuous unrealized loss position, 12 months or greater, gross unrealized loss	(157)	(353)
Continuous unrealized loss position, fair value	53,545	73,213
Continuous unrealized loss position, gross unrealized losses	$ (157)	$ (358)