Securities - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2019 USD ($) security	Dec. 31, 2018 USD ($) security	Dec. 31, 2017 USD ($) security
Debt Securities, Available-for-sale [Line Items]
Number of securities holdings | security	0	0
Shareholders equity, percentage	10.00%	10.00%
Amortized cost	$ 533,203,000	$ 521,337,000
Proceeds	11,403,000	0	$ 16,340,000
Tax provision related to net realized gains	31,000	0	540,000
Proceeds from sale of trading securities	301,000	455,000	1,091,000
Net realized gains on trading securities	16,000	151,000	93,000
Realized gains on Visa Class B shares	$ 463,000	0	$ 0
Other
Debt Securities, Available-for-sale [Line Items]
Number of securities holdings | security	1
Amortized cost	$ 1,020,000	1,020,000
Pooled trust preferred
Debt Securities, Available-for-sale [Line Items]
Number of securities holdings | security			2
Amortized cost			$ 800,000
Proceeds			$ 1,383,000
Collateral Pledged
Debt Securities, Available-for-sale [Line Items]
Securities pledged to public deposits	$ 268,447,000	$ 290,717,000