Investment Securities - Amortized Cost and Estimated Fair Value of Debt Securities by Contractual Maturity (Detail) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Amortized Cost
Due in one year	$ 1,093
Due after one year through five years	181,624
Due after five years through ten years	337,145
Due after ten years	1,313,468
Amortized Cost	1,833,330	$ 1,395,081
Estimated Fair Value
Due in one year	1,117
Due after one year through five years	181,791
Due after five years through ten years	338,985
Due after ten years	1,325,675
Totals	1,847,568	1,414,264
Amortized Cost
Due in one year	0
Due after one year through five years	1,011
Due after five years through ten years	20,641
Due after ten years	214,126
Totals	235,778	284,563
Estimated Fair Value
Due in one year	0
Due after one year through five years	1,125
Due after five years through ten years	21,363
Due after ten years	224,531
Totals	$ 247,019	$ 298,726