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Investments - Amortized Cost and Fair Value of Investments With Contractual Maturities (Detail) - USD ($)
$ in Thousands
Mar. 31, 2021
Dec. 31, 2020
Amortized Cost    
Due in one year or less $ 19,275  
Due after one year through five years 516,595  
Due after five years through ten years 374,044  
Due after ten years 21,095  
Perpetual maturity securities 165  
Amortized Cost 931,174 $ 815,647
Fair Value    
Due in one year or less 19,357  
Due after one year through five years 513,373  
Due after five years through ten years 359,574  
Due after ten years 20,630  
Perpetual maturity securities 197  
Fair Value $ 913,131 $ 819,861