Line items represent reporting concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Investment Securities - Additional Information (Detail) - USD ($)	3 Months Ended	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Jan. 01, 2019	Jan. 01, 2018
Investment [Line Items]
Unrealized gain on transfer of securities	$ 20,800,000
Other comprehensive income (loss), transfers from held-to-maturity to available-for-sale securities, before tax	291,000
Securities, carrying value	581,100,000	$ 581,100,000	$ 476,300,000
Equity securities, at fair value	16,473,000	16,473,000	15,921,000
Cumulative adjustment for adoption					$ 125,000	$ 0
Proceeds from sales of equity securities		1,287,000	2,155,000	$ 0
Gain (loss) on equity securities		496,000	(583,000)	$ 0
Investment in community development loans	3,500,000	$ 3,500,000
Redemption of funds		60 days
Unfunded commitments	0	$ 0
Fair value of securities			$ 11,300,000
Retained Earnings
Investment [Line Items]
Cumulative adjustment for adoption					$ 125,000	2,043,000
Other Financial Institutions
Investment [Line Items]
Equity securities, at fair value	1,700,000	1,700,000
Unfunded commitments	0	0
Community Reinvestment Funds
Investment [Line Items]
Equity securities, at fair value	$ 14,800,000	$ 14,800,000
Accounting Standards Update 2016-01 \| Retained Earnings
Investment [Line Items]
Cumulative adjustment for adoption						$ 2,000,000.0