Investment Securities - Additional Information (Detail) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Investment Holdings [Line Items]
Proceeds from sale of debt securities, available-for-sale	$ 0	$ 0	$ 94,696,000	$ 0
Debt securities, available-for-sale, realized gain			569,000
Debt securities, available-for-sale, realized loss			(227,000)
Securities, carrying value	578,500,000		578,500,000		$ 581,100,000
Equity securities, at fair value	14,076,000		14,076,000		$ 16,473,000
Payments to acquire equity securities, FV-NI			1,228,000	259,000
Proceeds from sales of equity securities			3,000,000	1,153,000
(Loss) gain on equity securities	(170,000)	$ 72,000	(625,000)	$ 525,000
Investment in community development loans	3,500,000		$ 3,500,000
Redemption of funds			60 days
Unfunded commitments	0		$ 0
Investment in government guaranteed loans, mortgage-backed securities, small business loans	9,700,000		9,700,000
Other financial institutions
Investment Holdings [Line Items]
Equity securities, at fair value	898,000		898,000
Unfunded commitments	0		0
Community reinvestment funds
Investment Holdings [Line Items]
Equity securities, at fair value	$ 13,200,000		$ 13,200,000