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Summary of Cash, Cash Equivalents and Marketable Securities (Detail) - USD ($)
$ in Thousands
Dec. 31, 2018
Dec. 31, 2017
Cash Cash Equivalents And Marketable Securities [Line Items]    
Amortized Cost $ 145,604 $ 191,934
Unrealized Losses (68) (80)
Estimated Fair Value 145,536 191,854
Cash and Cash Equivalents    
Cash Cash Equivalents And Marketable Securities [Line Items]    
Amortized Cost 49,348 26,584
Estimated Fair Value 49,348 26,584
Marketable Securities Available-for-Sale    
Cash Cash Equivalents And Marketable Securities [Line Items]    
Amortized Cost 96,256 165,350
Unrealized Losses (68) (80)
Estimated Fair Value 96,188 165,270
Cash | Cash and Cash Equivalents    
Cash Cash Equivalents And Marketable Securities [Line Items]    
Amortized Cost 3,147 4,041
Estimated Fair Value 3,147 4,041
Money Market Funds | Cash and Cash Equivalents    
Cash Cash Equivalents And Marketable Securities [Line Items]    
Amortized Cost 44,002 22,543
Estimated Fair Value 44,002 22,543
Corporate Debt Securities | Cash and Cash Equivalents    
Cash Cash Equivalents And Marketable Securities [Line Items]    
Amortized Cost 2,199  
Estimated Fair Value 2,199  
Corporate Debt Securities | Marketable Securities Available-for-Sale    
Cash Cash Equivalents And Marketable Securities [Line Items]    
Amortized Cost 39,108 32,931
Unrealized Losses (16) (15)
Estimated Fair Value 39,092 32,916
U.S. Treasuries | Marketable Securities Available-for-Sale    
Cash Cash Equivalents And Marketable Securities [Line Items]    
Amortized Cost 14,732 45,559
Unrealized Losses (8) (25)
Estimated Fair Value 14,724 45,534
U.S. Government Agency Securities | Marketable Securities Available-for-Sale    
Cash Cash Equivalents And Marketable Securities [Line Items]    
Amortized Cost 42,416 86,860
Unrealized Losses (44) (40)
Estimated Fair Value $ 42,372 $ 86,820