Loans - Allowance for Loan Losses (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for loan losses, Beginning Balance				$ 16,330				$ 16,737	$ 16,330	$ 16,737	$ 17,373
Charge-offs									(4,077)	(4,961)	(3,718)
Recoveries									785	405	522
Provision for loan losses	$ 3,105	$ 1,400	$ 1,134	1,016	$ 2,111	$ 622	$ 220	1,196	6,655	4,149	2,560
Allowance for loan losses, Ending Balance	19,693				16,330				19,693	16,330	16,737
Commercial, Industrial, and Agricultural [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for loan losses, Beginning Balance				5,428				6,035	5,428	6,035	7,114
Charge-offs									(544)	(601)	(307)
Recoveries									235	89	267
Provision for loan losses									1,041	(95)	(1,039)
Allowance for loan losses, Ending Balance	6,160				5,428				6,160	5,428	6,035
Commercial Mortgages [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for loan losses, Beginning Balance				6,753				5,605	6,753	5,605	5,310
Charge-offs									(116)	(201)	(486)
Recoveries									197	8	52
Provision for loan losses									2,173	1,341	729
Allowance for loan losses, Ending Balance	9,007				6,753				9,007	6,753	5,605
Residential Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for loan losses, Beginning Balance				1,653				2,475	1,653	2,475	2,479
Charge-offs									(466)	(499)	(632)
Recoveries									78	93	8
Provision for loan losses									768	(416)	620
Allowance for loan losses, Ending Balance	2,033				1,653				2,033	1,653	2,475
Consumer [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for loan losses, Beginning Balance				2,215				2,371	2,215	2,371	2,205
Charge-offs									(2,555)	(3,324)	(1,956)
Recoveries									161	122	96
Provision for loan losses									2,358	3,046	2,026
Allowance for loan losses, Ending Balance	2,179				2,215				2,179	2,215	2,371
Credit Cards [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for loan losses, Beginning Balance				93				90	93	90	71
Charge-offs									(144)	(96)	(116)
Recoveries									27	22	14
Provision for loan losses									144	77	121
Allowance for loan losses, Ending Balance	120				93				120	93	90
Overdrafts [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for loan losses, Beginning Balance				$ 188				$ 161	188	161	194
Charge-offs									(252)	(240)	(221)
Recoveries									87	71	85
Provision for loan losses									171	196	103
Allowance for loan losses, Ending Balance	$ 194				$ 188				$ 194	$ 188	$ 161