Summary of Company's Marketable Securities (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 650,578	$ 297,323
Gross Unrealized Gains	99	66
Gross Unrealized Losses	(751)	(86)
Fair Value	649,926	297,303
Commercial paper (Due within 1 year)
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	71,940	11,983
Gross Unrealized Losses	(25)	(5)
Fair Value	71,915	11,978
Corporate notes (Due within 1 year)
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	223,039	154,175
Gross Unrealized Gains	49	33
Gross Unrealized Losses	(137)	(46)
Fair Value	222,951	154,162
Corporate notes (Due after 1 year through 2 years)
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	289,659	74,312
Gross Unrealized Gains	32	23
Gross Unrealized Losses	(561)	(35)
Fair Value	289,130	74,300
U.S. Government obligations (Due within 1 year)
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	20,024	26,553
Gross Unrealized Gains	11	8
Fair Value	20,035	26,561
U.S. Government obligations (Due after 1 year through 2 years)
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	45,916	30,300
Gross Unrealized Gains	7	2
Gross Unrealized Losses	(28)
Fair Value	$ 45,895	$ 30,302