Investments - Fair Values and Gross Unrealized Losses of Available-for-Sale Securities Aggregated by Investment Category (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, continuous unrealized loss position, less than twelve months, fair value	$ 940,223	$ 522,536
Available-for-sale securities, continuous unrealized loss position, less than twelve months, gross unrealized losses	(3,301)	(1,592)
Available-for-sale securities, continuous unrealized loss position, twelve months or longer, fair value	164,429	47,940
Available-for-sale securities, continuous unrealized loss position, twelve months or longer, gross unrealized losses	(426)	(60)
Fair value	1,104,652	570,476
Gross unrealized losses	(3,727)	(1,652)
Commercial paper
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, continuous unrealized loss position, less than twelve months, fair value	14,809
Available-for-sale securities, continuous unrealized loss position, less than twelve months, gross unrealized losses	(5)
Available-for-sale securities, continuous unrealized loss position, twelve months or longer, fair value	0
Available-for-sale securities, continuous unrealized loss position, twelve months or longer, gross unrealized losses	0
Fair value	14,809
Gross unrealized losses	(5)
Corporate notes and bonds
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, continuous unrealized loss position, less than twelve months, fair value	819,113	492,503
Available-for-sale securities, continuous unrealized loss position, less than twelve months, gross unrealized losses	(2,703)	(1,530)
Available-for-sale securities, continuous unrealized loss position, twelve months or longer, fair value	141,874	47,940
Available-for-sale securities, continuous unrealized loss position, twelve months or longer, gross unrealized losses	(381)	(60)
Fair value	960,987	540,443
Gross unrealized losses	(3,084)	(1,590)
U.S. government agency securities
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, continuous unrealized loss position, less than twelve months, fair value	106,301	30,033
Available-for-sale securities, continuous unrealized loss position, less than twelve months, gross unrealized losses	(593)	(62)
Available-for-sale securities, continuous unrealized loss position, twelve months or longer, fair value	22,555	0
Available-for-sale securities, continuous unrealized loss position, twelve months or longer, gross unrealized losses	(45)	0
Fair value	128,856	30,033
Gross unrealized losses	$ (638)	$ (62)