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Investments - Summary of Fair Value of Available-for-Sale Investments by Type of Security (Detail) - USD ($)
$ in Thousands
Dec. 31, 2020
Dec. 31, 2019
Schedule Of Held To Maturity Securities [Line Items]    
Amortized Cost $ 230,351  
Unrealized Gains 64  
Fair Value 230,415  
Investments with a maturity of one year or less [Member]    
Schedule Of Held To Maturity Securities [Line Items]    
Amortized Cost 50,993 $ 313,537
Unrealized Gains 26 325
Fair Value 51,019 313,862
Investments with a maturity of one to two years [Member]    
Schedule Of Held To Maturity Securities [Line Items]    
Amortized Cost 179,358  
Unrealized Gains 38  
Fair Value 179,396  
U.S treasury bills [Member] | Investments with a maturity of one year or less [Member]    
Schedule Of Held To Maturity Securities [Line Items]    
Amortized Cost 29,997 232,336
Unrealized Gains 21 268
Fair Value 30,018 232,604
U.S treasury bills [Member] | Investments with a maturity of one to two years [Member]    
Schedule Of Held To Maturity Securities [Line Items]    
Amortized Cost 3,008  
Fair Value 3,008  
U.S agency securities [Member] | Investments with a maturity of one year or less [Member]    
Schedule Of Held To Maturity Securities [Line Items]    
Amortized Cost 20,996 81,201
Unrealized Gains 5 57
Fair Value 21,001 $ 81,258
U.S agency securities [Member] | Investments with a maturity of one to two years [Member]    
Schedule Of Held To Maturity Securities [Line Items]    
Amortized Cost 176,350  
Unrealized Gains 38  
Fair Value $ 176,388