Investment Securities - Additional Information (Detail) - USD ($)	1 Months Ended	3 Months Ended			9 Months Ended
	Mar. 31, 2023	Sep. 30, 2023	Mar. 31, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022	Dec. 31, 2022
Investment Holdings [Line Items]
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent		$ 9,694,000		$ 28,109,000	$ 11,365,000	$ 79,045,000
Proceeds from sale of debt securities, available-for-sale		0		0	0	0
Securities, carrying value		1,580,000,000			1,580,000,000		$ 1,340,000,000
Equity securities, at fair value		17,207,000			17,207,000		$ 17,283,000
Proceeds from sales of equity securities		0		0	0	0
Loss on equity securities		(294,000)		(464,000)	$ (281,000)	(1,313,000)
Redemption of funds, notice period					60 days
Unfunded commitments		0			$ 0
Investment in government guaranteed loans, mortgage-backed securities, small business loans		9,400,000			9,400,000
Charge-offs		0		0	6,640,000	0
(Benefit) provision for credit loss expense		0		1,363,000	6,330,000	4,082,000
Financing Receivable, Excluding Accrued Interest, Allowance for Credit Loss, Writeoff		(217,000)		(56,000)	(504,000)	(8,229,000)
Signature Bank
Investment Holdings [Line Items]
Charge-offs	$ 6,600,000		$ 6,600,000
(Benefit) provision for credit loss expense	$ 6,600,000		$ 6,600,000
Financial Asset Acquired with Credit Deterioration
Investment Holdings [Line Items]
Financing Receivable, Excluding Accrued Interest, Allowance for Credit Loss, Writeoff		0		0	0	(7,634,000)
Amortization of Gain on Debt Securities Reclassified to Held to Maturity
Investment Holdings [Line Items]
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent		111,000		$ 137,000	364,000	$ 420,000
Community reinvestment funds
Investment Holdings [Line Items]
Equity securities, at fair value		7,800,000			7,800,000
Other financial institutions
Investment Holdings [Line Items]
Unfunded commitments		$ 0			$ 0