Investments - Fair Values and Gross Unrealized Losses of Available-for-Sale Securities Aggregated by Investment Category (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, continuous unrealized loss position, less than twelve months, fair value	$ 522,536	$ 716,830
Available-for-sale securities, continuous unrealized loss position, less than twelve months, accumulated loss	(1,592)	(2,326)
Available-for-sale securities, continuous unrealized loss position, twelve months or longer, fair value	47,940	60,099
Available-for-sale securities, continuous unrealized loss position, twelve months or longer, accumulated loss	(60)	(130)
Fair value	570,476	776,929
Gross unrealized losses	(1,652)	(2,456)
Commercial paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, continuous unrealized loss position, less than twelve months, fair value		24,913
Available-for-sale securities, continuous unrealized loss position, less than twelve months, accumulated loss		(38)
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, accumulated loss		0
Fair value		24,913
Gross unrealized losses		(38)
Corporate notes and bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, continuous unrealized loss position, less than twelve months, fair value	492,503	539,586
Available-for-sale securities, continuous unrealized loss position, less than twelve months, accumulated loss	(1,530)	(1,897)
Available-for-sale securities, continuous unrealized loss position, twelve months or longer, fair value	47,940	60,099
Available-for-sale securities, continuous unrealized loss position, twelve months or longer, accumulated loss	(60)	(130)
Fair value	540,443	599,685
Gross unrealized losses	(1,590)	(2,027)
Certificates of deposit [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, continuous unrealized loss position, less than twelve months, fair value		19,750
Available-for-sale securities, continuous unrealized loss position, less than twelve months, accumulated loss		(17)
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, accumulated loss		0
Fair value		19,750
Gross unrealized losses		(17)
US government and agency securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, continuous unrealized loss position, less than twelve months, fair value	30,033	132,581
Available-for-sale securities, continuous unrealized loss position, less than twelve months, accumulated loss	(62)	(374)
Available-for-sale securities, continuous unrealized loss position, twelve months or longer, fair value	0	0
Available-for-sale securities, continuous unrealized loss position, twelve months or longer, accumulated loss	0	0
Fair value	30,033	132,581
Gross unrealized losses	$ (62)	$ (374)