Investment Securities - Maturity of Securities Available for Sale (Detail) - USD ($) $ in Thousands	Sep. 30, 2023	Dec. 31, 2022
Net Investment Income [Line Items]
Due within one year, Amortized Cost	$ 6,253
Due within one year, Estimated Fair Value	6,206
Due after one but within five years, Amortized Cost	8,890
Due after one but within five years, Estimated Fair Value	8,649
Due after five but within ten years, Amortized Cost	28,837
Due after five but within ten years, Estimated Fair Value	23,253
Due after ten years, Amortized Cost	0
Due after ten years, Estimated Fair Value	0
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, within One Year, Amortized Cost	43,980
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Fair Value	38,108
Debt Securities, Available-for-sale, Maturity, without Single Maturity Date, Amortized Cost	934,710
Debt Securities, Available-for-sale, Maturity, without Single Maturity Date, Fair Value	779,495
Amortized Cost	978,690	$ 898,702
Securities available for sale, at fair value	817,603	$ 762,661
Debt Securities [Member]
Net Investment Income [Line Items]
Amortized Cost	978,690
Securities available for sale, at fair value	817,603
US Government Agencies Debt Securities [Member]
Net Investment Income [Line Items]
Amortized Cost	433,000
Securities available for sale, at fair value	368,200
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Net Investment Income [Line Items]
Amortized Cost	501,700
Securities available for sale, at fair value	$ 411,300