Loans - Allowance for Loan Losses (Detail) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Allowance for loan losses, Beginning Balance	$ 19,704	$ 19,693	$ 16,330
Charge-offs	(6,751)	(6,593)	(4,077)
Recoveries	496	532	785
Provision for loan losses	6,024	6,072	6,655
Allowance for loan losses, Ending Balance	19,473	19,704	19,693
Commercial, industrial, and agricultural
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Allowance for loan losses, Beginning Balance	7,341	6,160	5,428
Charge-offs	(205)	(253)	(544)
Recoveries	17	171	235
Provision for loan losses	1,134	1,263	1,041
Allowance for loan losses, Ending Balance	8,287	7,341	6,160
Commercial mortgages
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Allowance for loan losses, Beginning Balance	7,490	9,007	6,753
Charge-offs	(3,391)	(3,337)	(116)
Recoveries	124	30	197
Provision for loan losses	2,729	1,790	2,173
Allowance for loan losses, Ending Balance	6,952	7,490	9,007
Residential real estate
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Allowance for loan losses, Beginning Balance	2,156	2,033	1,653
Charge-offs	(386)	(315)	(466)
Recoveries	73	67	78
Provision for loan losses	(344)	371	768
Allowance for loan losses, Ending Balance	1,499	2,156	2,033
Consumer
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Allowance for loan losses, Beginning Balance	2,377	2,179	2,215
Charge-offs	(2,200)	(2,279)	(2,555)
Recoveries	154	141	161
Provision for loan losses	2,080	2,336	2,358
Allowance for loan losses, Ending Balance	2,411	2,377	2,179
Credit cards
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Allowance for loan losses, Beginning Balance	103	120	93
Charge-offs	(116)	(90)	(144)
Recoveries	15	33	27
Provision for loan losses	82	40	144
Allowance for loan losses, Ending Balance	84	103	120
Overdrafts
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Allowance for loan losses, Beginning Balance	237	194	188
Charge-offs	(453)	(319)	(252)
Recoveries	113	90	87
Provision for loan losses	343	272	171
Allowance for loan losses, Ending Balance	$ 240	$ 237	$ 194

Amount of allowance for credit loss on financing receivable. Excludes allowance for financing receivable covered under loss sharing agreement.

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Amount of recovery of financing receivables doubtful of collection that were previously charged off.

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Amount of writeoff of financing receivable, charged against allowance for credit loss.

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Amount of credit loss expense (reversal of expense) for financing receivable.

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