Marketable Securities - Summary of Available-for-sale Investments (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 1,136,808	$ 1,265,713
Gross Unrealized Gains	631	827
Gross Unrealized Losses	(223)	(176)
Aggregate Fair Value	1,137,216	1,266,364
U.S. government and government agency debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	367,542	464,517
Gross Unrealized Gains	274	303
Gross Unrealized Losses	(9)	(5)
Aggregate Fair Value	367,807	464,815
Corporate debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	751,964	795,962
Gross Unrealized Gains	348	524
Gross Unrealized Losses	(214)	(170)
Aggregate Fair Value	752,098	796,316
Municipal securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	17,302	5,234
Gross Unrealized Gains	9
Gross Unrealized Losses		(1)
Aggregate Fair Value	$ 17,311	$ 5,233