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Investments - Summary of Gross Unrealized Losses and Fair Value of Available-For-Sale Investments in Unrealized Loss Position (Detail) - USD ($)
$ in Thousands
Dec. 31, 2019
Dec. 31, 2018
Schedule of Available-for-sale Securities [Line Items]    
Less Than 12 Months, Estimated Fair Value $ 220,834 $ 242,961
Less Than 12 Months, Unrealized Losses (51) (208)
12 Months or Greater, Estimated Fair Value   44,637
12 Months or Greater, Unrealized Losses   (214)
Total, Estimated Fair Value 220,834 287,598
Total, Unrealized Losses (51) (422)
U.S. Treasury notes    
Schedule of Available-for-sale Securities [Line Items]    
Less Than 12 Months, Estimated Fair Value 148,397  
Less Than 12 Months, Unrealized Losses (13)  
Total, Estimated Fair Value 148,397  
Total, Unrealized Losses (13)  
Corporate debt securities    
Schedule of Available-for-sale Securities [Line Items]    
Less Than 12 Months, Estimated Fair Value 11,334 91,265
Less Than 12 Months, Unrealized Losses (8) (130)
12 Months or Greater, Estimated Fair Value   44,637
12 Months or Greater, Unrealized Losses   (214)
Total, Estimated Fair Value 11,334 135,902
Total, Unrealized Losses (8) (344)
Government sponsored enterprise securities    
Schedule of Available-for-sale Securities [Line Items]    
Less Than 12 Months, Estimated Fair Value 61,103  
Less Than 12 Months, Unrealized Losses (30)  
Total, Estimated Fair Value 61,103  
Total, Unrealized Losses $ (30)  
Commercial paper    
Schedule of Available-for-sale Securities [Line Items]    
Less Than 12 Months, Estimated Fair Value   151,696
Less Than 12 Months, Unrealized Losses   (78)
Total, Estimated Fair Value   151,696
Total, Unrealized Losses   $ (78)