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Fair Value Measurements and Marketable Securities - Summary of Cash Equivalents and Marketable Securities (Detail) - USD ($)
$ in Thousands
Sep. 30, 2019
Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash equivalents $ 34,044 $ 45,210
Marketable securities 38,669 68,735
Total cash equivalents and marketable securities 72,713 113,945
Money Market Funds [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash equivalents 33,344 40,231
Commercial Paper [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash equivalents 700 4,979
Marketable securities 13,479 38,911
Corporate Bonds [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Marketable securities 13,169 25,830
U.S. Government Securities [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Marketable securities 12,021 3,994
Quoted Prices in Active Markets, (Level 1) [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash equivalents 33,344 40,231
Total cash equivalents and marketable securities 33,344 40,231
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 33,344 40,231
Significant Other Observable Inputs (Level 2) [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash equivalents 700 4,979
Marketable securities 38,669 68,735
Total cash equivalents and marketable securities 39,369 73,714
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 700 4,979
Marketable securities 13,479 38,911
Significant Other Observable Inputs (Level 2) [Member] | Corporate Bonds [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Marketable securities 13,169 25,830
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]    
Marketable securities $ 12,021 $ 3,994