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Fair Value Measurements and Marketable Securities - Summary of Cash Equivalents and Marketable Securities (Detail) - USD ($)
$ in Thousands
Jun. 30, 2018
Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash equivalents $ 36,251 $ 17,349
Marketable securities 35,104 61,275
Total cash equivalents and marketable securities 71,355 78,624
Money Market Funds [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash equivalents 22,971 15,802
Commercial Paper [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash equivalents 10,732 998
Marketable securities 21,000 9,951
U.S. Government Securities [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash equivalents 1,698  
Marketable securities 1,983  
Corporate Bonds [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash equivalents 850 549
Marketable securities 12,121 50,844
Certificates of Deposit [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Marketable securities   480
Quoted Prices in Active Markets, (Level 1) [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash equivalents 22,971 15,802
Total cash equivalents and marketable securities 22,971 15,802
Quoted Prices in Active Markets, (Level 1) [Member] | Money Market Funds [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash equivalents 22,971 15,802
Significant Other Observable Inputs (Level 2) [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash equivalents 13,280 1,547
Marketable securities 35,104 61,275
Total cash equivalents and marketable securities 48,384 62,822
Significant Other Observable Inputs (Level 2) [Member] | Commercial Paper [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash equivalents 10,732 998
Marketable securities 21,000 9,951
Significant Other Observable Inputs (Level 2) [Member] | U.S. Government Securities [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash equivalents 1,698  
Marketable securities 1,983  
Significant Other Observable Inputs (Level 2) [Member] | Corporate Bonds [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash equivalents 850 549
Marketable securities $ 12,121 50,844
Significant Other Observable Inputs (Level 2) [Member] | Certificates of Deposit [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Marketable securities   $ 480