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Marketable Securities - Summary of Fair Values and Unrealized Losses of Marketable Securities Held by the Company in an Unrealized Loss Position for Less Than 12 months and 12 Months or Greater (Detail) - USD ($)
$ in Thousands
Sep. 30, 2019
Dec. 31, 2018
Schedule Of Available For Sale Securities [Line Items]    
Less than 12 Months, Fair Value $ 27,851  
Less than 12 Months, Unrealized Losses (24) $ (391)
12 Months or Greater, Unrealized Losses   (23)
Total, Fair Value 27,851  
Total, Unrealized Losses (24) (414)
Less than 12 Months, Fair Value   298,407
12 Months or Greater, Fair Value   12,424
Total, Fair Value   310,831
U.S. Government and Federal Agency Securities [Member]    
Schedule Of Available For Sale Securities [Line Items]    
Less than 12 Months, Fair Value 4,497  
Less than 12 Months, Unrealized Losses (3) (37)
Total, Fair Value 4,497  
Total, Unrealized Losses (3) (37)
Less than 12 Months, Fair Value   53,124
Total, Fair Value   53,124
Corporate Bonds [Member]    
Schedule Of Available For Sale Securities [Line Items]    
Less than 12 Months, Fair Value 23,354  
Less than 12 Months, Unrealized Losses (21) (354)
12 Months or Greater, Unrealized Losses   (23)
Total, Fair Value 23,354  
Total, Unrealized Losses $ (21) (377)
Less than 12 Months, Fair Value   245,283
12 Months or Greater, Fair Value   12,424
Total, Fair Value   $ 257,707