Short-term and Long-term Investments - Summary of Amortized Cost, Gross Unrealized Gains and Losses and Fair Value of Short-term Investments (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 149,652	$ 188,540
Gross Unrealized Gains	116	35
Gross Unrealized Losses	0	(14)
Aggregate Fair Value	149,768	188,561
US Government Debt Securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	148,632	178,277
Gross Unrealized Gains	116	35
Gross Unrealized Losses	0	(13)
Aggregate Fair Value	148,748	178,299
Commercial Paper
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		7,997
Gross Unrealized Gains		0
Gross Unrealized Losses		0
Aggregate Fair Value		7,997
Certificates of Deposit
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,020	2,266
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	(1)
Aggregate Fair Value	$ 1,020	$ 2,265