Short-term Investments - Measured at Fair Value (Detail) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	$ 105,409	$ 111,832
Unrealized gains	7	3
Unrealized loss	(34)	(58)
Fair value	$ 105,382	111,777
Maximum
Schedule of Available-for-sale Securities [Line Items]
Maturity period of available for sale securities	1 year
Marketable securities
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	$ 98,409	108,832
Unrealized gains	7	3
Unrealized loss	(34)	(58)
Fair value	98,382	108,777
U S agency notes
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	36,617	33,489
Unrealized gains	4
Unrealized loss	(3)	(13)
Fair value	36,618	33,476
Corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	36,998	44,025
Unrealized gains	1	3
Unrealized loss	(31)	(44)
Fair value	36,968	43,984
Commercial paper
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	24,794	31,318
Unrealized gains	2
Unrealized loss		(1)
Fair value	24,796	31,317
Certificates of deposit
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	7,000	3,000
Fair value	$ 7,000	$ 3,000