Summary of Company's Marketable Debt Securities (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 905,822	$ 1,060,817
Gross Unrealized Gains	12	12
Gross Unrealized Losses	(612)	(1,653)
Fair Value	905,222	1,059,176
Certificate of deposit
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	18,800	30,200
Fair Value	18,800	30,200
Commercial paper
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	101,379	56,951
Gross Unrealized Losses	(3)
Fair Value	101,376	56,951
Corporate notes
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	324,615	373,736
Gross Unrealized Gains	11	11
Gross Unrealized Losses	(247)	(495)
Fair Value	324,379	373,252
U.S. government-sponsored enterprise securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	111,427	399,281
Gross Unrealized Losses	(88)	(983)
Fair Value	111,339	398,298
U.S. treasury securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	349,601	200,649
Gross Unrealized Gains	1	1
Gross Unrealized Losses	(274)	(175)
Fair Value	$ 349,328	$ 200,475