Investment Securities - Amortized Cost and Estimated Fair Value of Debt Securities by Contractual Maturity (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Amortized Cost
Due in one year	$ 20,000
Due after one year through five years	193,791
Due after five years through ten years	157,945
Due after ten years	1,023,345
Amortized Cost	1,395,081	$ 946,751
Estimated Fair Value
Due in one year	19,994
Due after one year through five years	194,337
Due after five years through ten years	159,670
Due after ten years	1,040,263
Totals	1,414,264	950,138
Amortized Cost
Due in one year	0
Due after one year through five years	993
Due after five years through ten years	18,755
Due after ten years	264,815
Totals	284,563	375,606
Estimated Fair Value
Due in one year	0
Due after one year through five years	1,135
Due after five years through ten years	19,640
Due after ten years	277,951
Totals	$ 298,726	$ 381,525