Investment Securities - Reconciliation of Available-for-Sale Securities (Detail) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 795,559	$ 752,260
Gross Unrealized Gains	18,597	5,785
Gross Unrealized Losses	(2,175)	(2,145)
Investment securities available for sale, at fair value	811,981	755,900
U.S. Treasury and U.S. government agencies
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	138,413	135,361
Gross Unrealized Gains	1,792	722
Gross Unrealized Losses	(347)	(436)
Investment securities available for sale, at fair value	139,858	135,647
Mortgage-backed securities, residential
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	448,806	500,245
Gross Unrealized Gains	11,891	3,185
Gross Unrealized Losses	(303)	(1,551)
Investment securities available for sale, at fair value	460,394	501,879
Mortgage-backed securities, multifamily
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	45,539	48,675
Gross Unrealized Gains	2,267	633
Gross Unrealized Losses	0	(123)
Investment securities available for sale, at fair value	47,806	49,185
Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	41,815	0
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(1,507)	0
Investment securities available for sale, at fair value	40,308	0
Obligations of states and political subdivisions
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	96,420	58,979
Gross Unrealized Gains	2,445	1,077
Gross Unrealized Losses	(16)	(35)
Investment securities available for sale, at fair value	98,849	60,021
Debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	24,566	9,000
Gross Unrealized Gains	202	168
Gross Unrealized Losses	(2)	0
Investment securities available for sale, at fair value	$ 24,766	$ 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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