Loans - Allowance for Loan Losses (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowances for loan losses, Beginning Balance				$ 17,373				$ 16,234	$ 17,373	$ 16,234	$ 14,060
Charge-offs									(3,718)	(3,182)	(5,630)
Recoveries									522	481	1,666
Provision for loan losses	$ 668	$ 463	$ 486	943	$ 282	$ 1,038	$ 1,501	1,019	2,560	3,840	6,138
Allowances for loan losses, Ending Balance	16,737				17,373				16,737	17,373	16,234
Commercial, Industrial, and Agricultural [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowances for loan losses, Beginning Balance				7,114				8,212	7,114	8,212	4,940
Charge-offs									(307)	(618)	(958)
Recoveries									267	1	7
Provision for loan losses									(1,039)	(481)	4,223
Allowances for loan losses, Ending Balance	6,035				7,114				6,035	7,114	8,212
Commercial Mortgages [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowances for loan losses, Beginning Balance				5,310				3,536	5,310	3,536	4,697
Charge-offs									(486)	(50)	(1,931)
Recoveries									52	210	1,430
Provision for loan losses									729	1,614	(660)
Allowances for loan losses, Ending Balance	5,605				5,310				5,605	5,310	3,536
Residential Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowances for loan losses, Beginning Balance				2,479				2,450	2,479	2,450	2,466
Charge-offs									(632)	(436)	(467)
Recoveries									8	41	5
Provision for loan losses									620	424	446
Allowances for loan losses, Ending Balance	2,475				2,479				2,475	2,479	2,450
Consumer [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowances for loan losses, Beginning Balance				2,205				1,763	2,205	1,763	1,699
Charge-offs									(1,956)	(1,744)	(1,919)
Recoveries									96	93	114
Provision for loan losses									2,026	2,093	1,869
Allowances for loan losses, Ending Balance	2,371				2,205				2,371	2,205	1,763
Credit Cards [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowances for loan losses, Beginning Balance				71				66	71	66	83
Charge-offs									(116)	(78)	(97)
Recoveries									14	25	16
Provision for loan losses									121	58	64
Allowances for loan losses, Ending Balance	90				71				90	71	66
Overdrafts [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowances for loan losses, Beginning Balance				$ 194				$ 207	194	207	175
Charge-offs									(221)	(256)	(258)
Recoveries									85	111	94
Provision for loan losses									103	132	196
Allowances for loan losses, Ending Balance	$ 161				$ 194				$ 161	$ 194	$ 207