Fair Value Measurements - Fair Value of Financial Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Financial assets	$ 100,458	$ 44,337
Fair Value Measurements Using Level 1
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Financial assets	26,439	19,787
Fair Value Measurements Using Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Financial assets	74,019	24,550
Money Market Funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Financial assets	26,439	19,787
Money Market Funds | Fair Value Measurements Using Level 1
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Financial assets	26,439	19,787
Commercial Paper
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Financial assets	29,428	3,996
Commercial Paper | Fair Value Measurements Using Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Financial assets	29,428	3,996
Corporate Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Financial assets	19,578	16,540
Corporate Securities | Fair Value Measurements Using Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Financial assets	19,578	16,540
Agency Discount Instruments
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Financial assets	2,998
Agency Discount Instruments | Fair Value Measurements Using Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Financial assets	2,998
U.S. Government Agency Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Financial assets	22,015	4,014
U.S. Government Agency Securities | Fair Value Measurements Using Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Financial assets	$ 22,015	$ 4,014