Investment Securities - Amortized Cost and Estimated Fair Value of Debt Securities Available for Sale (Detail) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Net Investment Income [Line Items]
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Fair Value	$ 138,600
Due after five but within ten years, Amortized Cost	7,127
Due after one but within five years, Amortized Cost	20,225
Debt Securities, Available-for-sale, Maturity, without Single Maturity Date, Amortized Cost	743,523
Due within one year, Amortized Cost	136,893
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, within One Year, Amortized Cost	104,991
Due after ten years, Amortized Cost	4,550
Corporate/Mortgage-Backed Securities, Amortized cost	769,636
Amortized Cost	880,416	$ 896,495
Due within one year, Estimated Fair Value	105,041
Due after one but within five years, Estimated Fair Value	21,260
Due after five but within ten years, Estimated Fair Value	7,749
Due after ten years, Estimated Fair Value	4,550
Securities available for sale, at fair value	908,236	902,292
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	27,832	8,906
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	12	3,109
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Net Investment Income [Line Items]
Amortized Cost	527,600
Securities available for sale, at fair value	543,500
Debt Securities [Member]
Net Investment Income [Line Items]
Amortized Cost	880,416
Securities available for sale, at fair value	908,236
Corporate Debt Securities [Member]
Net Investment Income [Line Items]
Securities available for sale, at fair value	24,440	23,962
Obligations of States and Political Subdivisions [Member]
Net Investment Income [Line Items]
Securities available for sale, at fair value	13,167	17,909
Obligations of U.S. Government-Sponsored Enterprises - Other Government-Sponsored Enterprises [Member]
Net Investment Income [Line Items]
Securities available for sale, at fair value	100,993	1,000
Residential Mortgage Backed Securities [Member]
Net Investment Income [Line Items]
Securities available for sale, at fair value	543,543	$ 661,947
US Government Agencies Debt Securities [Member]
Net Investment Income [Line Items]
Amortized Cost	216,000
Securities available for sale, at fair value	$ 226,100

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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