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Fair Value Measurements - Summary of Financial Assets Recognized at Fair Value on Recurring Basis (Detail) - Fair Value on Recurring Basis [Member] - USD ($)
$ in Thousands
Dec. 31, 2019
Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash equivalents $ 46,917 $ 45,986
Marketable securities:    
Marketable securities 227,246 255,203
Total 274,163 301,189
U.S. Treasury Securities [Member]    
Marketable securities:    
Marketable securities 159,502 165,948
Financial Institution Debt Securities [Member]    
Marketable securities:    
Marketable securities 40,278 65,420
Corporate Debt Securities [Member]    
Marketable securities:    
Marketable securities 18,967 23,835
Other Asset Backed Securities [Member]    
Marketable securities:    
Marketable securities 8,499  
Level 1 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash equivalents 46,917 45,986
Marketable securities:    
Marketable securities 159,502 165,948
Total 206,419 211,934
Level 1 [Member] | U.S. Treasury Securities [Member]    
Marketable securities:    
Marketable securities 159,502 165,948
Level 2 [Member]    
Marketable securities:    
Marketable securities 67,744 89,255
Total 67,744 89,255
Level 2 [Member] | Financial Institution Debt Securities [Member]    
Marketable securities:    
Marketable securities 40,278 65,420
Level 2 [Member] | Corporate Debt Securities [Member]    
Marketable securities:    
Marketable securities 18,967 $ 23,835
Level 2 [Member] | Other Asset Backed Securities [Member]    
Marketable securities:    
Marketable securities $ 8,499