Investment Securities - Reconciliation of Available-for-Sale Securities (Detail) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 767,558	$ 752,260
Gross Unrealized Gains	17,439	5,785
Gross Unrealized Losses	(1,666)	(2,145)
Investment securities available for sale, at fair value	783,331	755,900
U.S. Treasury and U.S. government agencies
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	71,953	135,361
Gross Unrealized Gains	1,640	722
Gross Unrealized Losses	(318)	(436)
Investment securities available for sale, at fair value	73,275	135,647
Mortgage-backed securities, residential
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	421,453	500,245
Gross Unrealized Gains	10,583	3,185
Gross Unrealized Losses	(246)	(1,551)
Investment securities available for sale, at fair value	431,790	501,879
Mortgage-backed securities, multifamily
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	42,943	48,675
Gross Unrealized Gains	2,107	633
Gross Unrealized Losses	(13)	(123)
Investment securities available for sale, at fair value	45,037	49,185
Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	41,354	0
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(643)	0
Investment securities available for sale, at fair value	40,711	0
Obligations of states and political subdivisions
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	162,296	58,979
Gross Unrealized Gains	2,859	1,077
Gross Unrealized Losses	(445)	(35)
Investment securities available for sale, at fair value	164,710	60,021
Debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	27,559	9,000
Gross Unrealized Gains	250	168
Gross Unrealized Losses	(1)	0
Investment securities available for sale, at fair value	$ 27,808	$ 9,168

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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