Investments - Summary of Investments (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 523,849	$ 231,889
Gross Unrealized Gains	139	8
Gross Unrealized Losses	(381)	(113)
Estimated Fair Value	523,607	231,784
Available-for-sale Securities, Debt Maturities, Next Twelve Months, Fair Value	268,251
Available-for-sale Securities, Debt Maturities, Year Two Through Three, Fair Value	255,356
Commercial paper
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	124,330	72,850
Gross Unrealized Gains	10	0
Gross Unrealized Losses	(21)	(15)
Estimated Fair Value	124,319	72,835
Corporate notes and bonds securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	399,519	158,038
Gross Unrealized Gains	129	8
Gross Unrealized Losses	(360)	(98)
Estimated Fair Value	399,288	157,948
US government agencies securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		1,001
Gross Unrealized Gains		0
Gross Unrealized Losses		0
Estimated Fair Value		$ 1,001