Investments - Amortized Cost and Fair Value of Investments With Contractual Maturities (Detail) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Amortized Cost
Due in one year or less	$ 13,195
Due after one year through five years	539,113
Due after five years through ten years	353,906
Due after ten years	22,812
Perpetual maturity securities	165
Amortized Cost	929,191	$ 815,647
Fair Value
Due in one year or less	13,319
Due after one year through five years	537,491
Due after five years through ten years	347,811
Due after ten years	22,967
Perpetual maturity securities	212
Fair Value	$ 921,800	$ 819,861