Loans - Allowance for Loan Losses (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for loan losses, Beginning Balance	$ 19,693	$ 16,330	$ 16,737
Charge-offs	(6,593)	(4,077)	(4,961)
Recoveries	532	785	405
Provision for loan losses	6,072	6,655	4,149
Allowance for loan losses, Ending Balance	19,704	19,693	16,330
Commercial, industrial, and agricultural
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for loan losses, Beginning Balance	6,160	5,428	6,035
Charge-offs	(253)	(544)	(601)
Recoveries	171	235	89
Provision for loan losses	1,263	1,041	(95)
Allowance for loan losses, Ending Balance	7,341	6,160	5,428
Commercial mortgages
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for loan losses, Beginning Balance	9,007	6,753	5,605
Charge-offs	(3,337)	(116)	(201)
Recoveries	30	197	8
Provision for loan losses	1,790	2,173	1,341
Allowance for loan losses, Ending Balance	7,490	9,007	6,753
Residential real estate
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for loan losses, Beginning Balance	2,033	1,653	2,475
Charge-offs	(315)	(466)	(499)
Recoveries	67	78	93
Provision for loan losses	371	768	(416)
Allowance for loan losses, Ending Balance	2,156	2,033	1,653
Consumer
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for loan losses, Beginning Balance	2,179	2,215	2,371
Charge-offs	(2,279)	(2,555)	(3,324)
Recoveries	141	161	122
Provision for loan losses	2,336	2,358	3,046
Allowance for loan losses, Ending Balance	2,377	2,179	2,215
Credit cards
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for loan losses, Beginning Balance	120	93	90
Charge-offs	(90)	(144)	(96)
Recoveries	33	27	22
Provision for loan losses	40	144	77
Allowance for loan losses, Ending Balance	103	120	93
Overdrafts
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for loan losses, Beginning Balance	194	188	161
Charge-offs	(319)	(252)	(240)
Recoveries	90	87	71
Provision for loan losses	272	171	196
Allowance for loan losses, Ending Balance	$ 237	$ 194	$ 188