Marketable Securities - Summary of Available-for-Sale Investments (Detail) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 411,104	$ 1,016,999
Gross Unrealized Gains	136	107
Gross Unrealized Losses	(24)	(500)
Aggregate Fair Value	411,216	1,016,606
U.S. government and government agency debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	196,828	405,049
Gross Unrealized Gains	86	68
Gross Unrealized Losses	(1)	(135)
Aggregate Fair Value	196,913	404,982
Corporate debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	214,276	611,950
Gross Unrealized Gains	50	39
Gross Unrealized Losses	(23)	(365)
Aggregate Fair Value	$ 214,303	$ 611,624