Short-term and Long-term Investments - Summary of Amortized Cost, Gross Unrealized Gains and Losses and Fair Value of Short-term and Long-term Investments (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 115,566	$ 188,540
Gross Unrealized Gains	54	35
Gross Unrealized Losses	(12)	(14)
Aggregate Fair Value	115,608	188,561
U.S. government debt securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	115,293	178,277
Gross Unrealized Gains	54	35
Gross Unrealized Losses	(12)	(13)
Aggregate Fair Value	115,335	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	273	2,266
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	(1)
Aggregate Fair Value	$ 273	$ 2,265