Short-term Investments - Measured at Fair Value (Detail) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	$ 87,621	$ 111,832
Unrealized gains	5	3
Unrealized loss	(11)	(58)
Fair value	$ 87,615	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	$ 72,621	108,832
Unrealized gains	5	3
Unrealized loss	(11)	(58)
Fair value	72,615	108,777
U S agency notes
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	25,281	33,489
Unrealized gains	3
Unrealized loss	(1)	(13)
Fair value	25,283	33,476
Corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	27,027	44,025
Unrealized gains	1	3
Unrealized loss	(10)	(44)
Fair value	27,018	43,984
Commercial paper
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	20,313	31,318
Unrealized gains	1
Unrealized loss		(1)
Fair value	20,314	31,317
Certificates of deposit
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	15,000	3,000
Fair value	$ 15,000	$ 3,000