Investment Securities - Maturity of Securities Available for Sale (Detail) - USD ($) $ in Thousands	Mar. 31, 2024	Dec. 31, 2023
Net Investment Income [Line Items]
Due within one year, Amortized Cost	$ 255
Due within one year, Estimated Fair Value	255
Due after one but within five years, Amortized Cost	18,023
Due after one but within five years, Estimated Fair Value	17,865
Due after five but within ten years, Amortized Cost	58,952
Due after five but within ten years, Estimated Fair Value	53,905
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	77,230
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Fair Value	72,025
Debt Securities, Available-for-sale, Maturity, without Single Maturity Date, Amortized Cost	1,073,794
Debt Securities, Available-for-sale, Maturity, without Single Maturity Date, Fair Value	951,686
Amortized Cost	1,151,024	$ 1,138,425
Securities available for sale, at fair value	1,023,711	$ 1,020,986
Debt Securities [Member]
Net Investment Income [Line Items]
Amortized Cost	1,151,024
Securities available for sale, at fair value	1,023,711
US Government Agencies Debt Securities [Member]
Net Investment Income [Line Items]
Amortized Cost	529,900
Securities available for sale, at fair value	477,500
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Net Investment Income [Line Items]
Amortized Cost	543,900
Securities available for sale, at fair value	$ 474,200