Investment Securities - Amortized Cost and Estimated Fair Value of Debt Securities by Contractual Maturity (Detail) - USD ($) $ in Thousands	Sep. 30, 2023	Dec. 31, 2022
Amortized Cost
Due in one year	$ 86,722
Due after one year through five years	62,069
Due after five years through ten years	398,920
Due after ten years	1,961,912
Amortized Cost	2,509,623	$ 2,742,987
Estimated Fair Value
Due in one year	84,609
Due after one year through five years	58,068
Due after five years through ten years	377,030
Due after ten years	1,654,630
Totals	2,174,337	2,452,438
Amortized Cost
Due in one year	0
Due after one year through five years	6,302
Due after five years through ten years	80,279
Due after ten years	52,477
Totals	139,058	160,983
Estimated Fair Value
Due in one year	0
Due after one year through five years	6,015
Due after five years through ten years	71,376
Due after ten years	46,667
Totals	$ 124,058	$ 149,938