Investment Securities - Amortized Cost and Estimated Fair Value of Debt Securities by Contractual Maturity (Detail) - USD ($) $ in Thousands	Mar. 31, 2024	Dec. 31, 2023
Amortized Cost
Due in one year	$ 51,718
Due after one year through five years	49,150
Due after five years through ten years	371,852
Due after ten years	1,848,888
Amortized Cost	2,321,608	$ 2,384,325
Estimated Fair Value
Due in one year	51,226
Due after one year through five years	46,062
Due after five years through ten years	358,110
Due after ten years	1,618,490
Totals	2,073,888	2,152,504
Amortized Cost
Due in one year	0
Due after one year through five years	5,396
Due after five years through ten years	92,951
Due after ten years	29,464
Totals	127,811	133,494
Estimated Fair Value
Due in one year	0
Due after one year through five years	5,206
Due after five years through ten years	86,475
Due after ten years	26,847
Totals	$ 118,528	$ 125,126