Investments - Fair Values and Gross Unrealized Losses of Available-for-Sale Securities Aggregated by Investment Category (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]
Continuous loss position, less than 12 months, fair value	$ 727,361	$ 940,223
Continuous loss position, less than 12 months, gross unrealized losses	(2,608)	(3,301)
Continuous loss position, 12 months or greater, fair value	356,413	164,429
Continuous loss position, 12 months or greater, gross unrealized losses	(1,968)	(426)
Continuous loss position, total, fair value	1,083,774	1,104,652
Continuous loss position, total, gross unrealized losses	(4,576)	(3,727)
Commercial paper
Debt Securities, Available-for-sale [Line Items]
Continuous loss position, less than 12 months, fair value		14,809
Continuous loss position, less than 12 months, gross unrealized losses		(5)
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		14,809
Continuous loss position, total, gross unrealized losses		(5)
Corporate notes and bonds
Debt Securities, Available-for-sale [Line Items]
Continuous loss position, less than 12 months, fair value	714,605	819,113
Continuous loss position, less than 12 months, gross unrealized losses	(2,603)	(2,703)
Continuous loss position, 12 months or greater, fair value	294,956	141,874
Continuous loss position, 12 months or greater, gross unrealized losses	(1,615)	(381)
Continuous loss position, total, fair value	1,009,561	960,987
Continuous loss position, total, gross unrealized losses	(4,218)	(3,084)
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	11,756	106,301
Continuous loss position, less than 12 months, gross unrealized losses	(5)	(593)
Continuous loss position, 12 months or greater, fair value	61,457	22,555
Continuous loss position, 12 months or greater, gross unrealized losses	(353)	(45)
Continuous loss position, total, fair value	73,213	128,856
Continuous loss position, total, gross unrealized losses	$ (358)	$ (638)