Loans - Allowance for Loan Losses (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for loan losses, Beginning Balance	$ 22,122	$ 17,269	$ 19,693	$ 16,330
Charge-offs	(813)	(977)	(2,249)	(2,672)
Recoveries	106	157	435	641
Provision for loan losses	1,095	1,400	4,631	3,550
Allowance for loan losses, Ending Balance	22,510	17,849	22,510	17,849
Commercial, Industrial, and Agricultural
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for loan losses, Beginning Balance	7,143	5,563	6,160	5,428
Charge-offs	(30)	(20)	(61)	(50)
Recoveries	3	36	165	167
Provision for loan losses	(536)	(223)	316	(189)
Allowance for loan losses, Ending Balance	6,580	5,356	6,580	5,356
Commercial mortgages
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for loan losses, Beginning Balance	10,615	7,641	9,007	6,753
Charge-offs	0	(22)	0	(22)
Recoveries	0	3	0	197
Provision for loan losses	682	472	2,290	1,166
Allowance for loan losses, Ending Balance	11,297	8,094	11,297	8,094
Residential Real Estate
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for loan losses, Beginning Balance	1,900	1,670	2,033	1,653
Charge-offs	(212)	(130)	(289)	(328)
Recoveries	55	0	67	73
Provision for loan losses	235	468	167	610
Allowance for loan losses, Ending Balance	1,978	2,008	1,978	2,008
Consumer
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for loan losses, Beginning Balance	2,156	2,068	2,179	2,215
Charge-offs	(469)	(703)	(1,610)	(1,969)
Recoveries	28	96	112	110
Provision for loan losses	608	627	1,642	1,732
Allowance for loan losses, Ending Balance	2,323	2,088	2,323	2,088
Credit Cards
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for loan losses, Beginning Balance	101	142	120	93
Charge-offs	(8)	(39)	(53)	(111)
Recoveries	3	8	27	23
Provision for loan losses	11	0	13	106
Allowance for loan losses, Ending Balance	107	111	107	111
Overdrafts
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for loan losses, Beginning Balance	207	185	194	188
Charge-offs	(94)	(63)	(236)	(192)
Recoveries	17	14	64	71
Provision for loan losses	95	56	203	125
Allowance for loan losses, Ending Balance	$ 225	$ 192	$ 225	$ 192