Short-term Investments - Summary of Investments in Continuous Unrealized Loss Position for Less Than 12 Months (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Fair value	$ 35,627
Less Than 12 Months, Gross unrealized losses	(45)
U.S. government-related debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Fair value	2,996
Less Than 12 Months, Gross unrealized losses	(4)
Corporate debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Fair value	30,372
Less Than 12 Months, Gross unrealized losses	(40)
Asset-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Fair value	2,259
Less Than 12 Months, Gross unrealized losses	$ (1)