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Investments - Summary of Gross Unrealized Losses and Fair Value of Available-For-Sale Investment Securities in Unrealized Loss Position (Detail) - USD ($)
$ in Thousands
Jun. 30, 2020
Dec. 31, 2019
Schedule of Available-for-sale Securities [Line Items]    
Less Than 12 Months, Estimated Fair Value   $ 220,834
Less Than 12 Months, Unrealized Losses   (51)
12 Months or Greater, Estimated Fair Value $ 4,003  
12 Months or Greater, Unrealized Losses (1)  
Total, Estimated Fair Value 4,003 220,834
Total, Unrealized Losses (1) (51)
Corporate debt securities    
Schedule of Available-for-sale Securities [Line Items]    
Less Than 12 Months, Estimated Fair Value   11,334
Less Than 12 Months, Unrealized Losses   (8)
12 Months or Greater, Estimated Fair Value 4,003  
12 Months or Greater, Unrealized Losses (1)  
Total, Estimated Fair Value 4,003 11,334
Total, Unrealized Losses $ (1) (8)
US Treasury Securities [Member]    
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)
US Government Sponsored Enterprises Debt Securities [Member]    
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)