Investment Securities - Amortized Cost and Estimated Fair Value of Debt Securities by Contractual Maturity (Detail) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Amortized Cost
Due in one year	$ 8,771
Due after one year through five years	154,411
Due after five years through ten years	337,783
Due after ten years	1,586,922
Amortized Cost	2,087,887	$ 1,395,081
Estimated Fair Value
Due in one year	8,872
Due after one year through five years	154,531
Due after five years through ten years	339,617
Due after ten years	1,592,801
Totals	2,095,821	1,414,264
Amortized Cost
Due in one year	0
Due after one year through five years	1,020
Due after five years through ten years	22,548
Due after ten years	193,411
Totals	216,979	284,563
Estimated Fair Value
Due in one year	0
Due after one year through five years	1,129
Due after five years through ten years	23,346
Due after ten years	203,699
Totals	$ 228,174	$ 298,726