Fair Value Measurements and Marketable Securities - Financial Assets at Fair Value on Recurring Basis (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total cash equivalents and marketable securities	$ 68,305	$ 82,033
Basis of Fair Value Measurements, Level 1
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total cash equivalents and marketable securities	6,175	4,285
Basis of Fair Value Measurements, Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total cash equivalents and marketable securities	62,130	77,748
Basis of Fair Value Measurements, Level 3
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total cash equivalents and marketable securities	0	0
Money Market Funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total cash equivalents and marketable securities	6,175	4,285
Money Market Funds | Basis of Fair Value Measurements, Level 1
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total cash equivalents and marketable securities	6,175	4,285
Money Market Funds | Basis of Fair Value Measurements, Level 3
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total cash equivalents and marketable securities	0	0
Certificates of Deposit
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total cash equivalents and marketable securities	2,554	1,584
Certificates of Deposit | Basis of Fair Value Measurements, Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total cash equivalents and marketable securities	2,554	1,584
Certificates of Deposit | Basis of Fair Value Measurements, Level 3
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total cash equivalents and marketable securities	0	0
Corporate Debt Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total cash equivalents and marketable securities	33,956	49,019
Corporate Debt Securities | Basis of Fair Value Measurements, Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total cash equivalents and marketable securities	33,956	49,019
Corporate Debt Securities | Basis of Fair Value Measurements, Level 3
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total cash equivalents and marketable securities	0	0
U. S. Government Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total cash equivalents and marketable securities	20,046	21,731
U. S. Government Securities | Basis of Fair Value Measurements, Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total cash equivalents and marketable securities	20,046	21,731
U. S. Government Securities | Basis of Fair Value Measurements, Level 3
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total cash equivalents and marketable securities	0	0
Municipal Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total cash equivalents and marketable securities	227	2,815
Municipal Securities | Basis of Fair Value Measurements, Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total cash equivalents and marketable securities	227	2,815
Municipal Securities | Basis of Fair Value Measurements, Level 3
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total cash equivalents and marketable securities	0	0
Commercial Paper
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total cash equivalents and marketable securities	5,347	2,599
Commercial Paper | Basis of Fair Value Measurements, Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total cash equivalents and marketable securities	5,347	2,599
Commercial Paper | Basis of Fair Value Measurements, Level 3
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total cash equivalents and marketable securities	$ 0	$ 0