Allowance for Loan Losses and Recorded Investment in Loans by Portfolio Class (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010	Dec. 31, 2009
Allowance for Loan Losses:
Individually Evaluated for Impairment	$ 5,323		$ 4,834
Collectively Evaluated for Impairment	10,109		10,401
Acquired with Deteriorated Credit Quality	88		77
Total Ending Allowance Balance	15,520		15,312		13,317	11,016
Loans:
Loans Individually Evaluated for Impairment	12,520		16,613
Loans Collectively Evaluated for Impairment	1,189,729		1,096,571
Loans Acquired with Deteriorated Credit Quality	11,174		16,121
Total Ending Loans Balance (1)	1,213,423	[1]	1,129,305	[2]
Commercial and Industrial Loans and Leases [Member]
Allowance for Loan Losses:
Individually Evaluated for Impairment	1,279		466
Collectively Evaluated for Impairment	3,208		3,027
Acquired with Deteriorated Credit Quality	68		0
Total Ending Allowance Balance	4,555		3,493		3,713
Loans:
Loans Individually Evaluated for Impairment	2,547		3,567
Loans Collectively Evaluated for Impairment	331,920		287,924
Loans Acquired with Deteriorated Credit Quality	1,840		2,596
Total Ending Loans Balance (1)	336,307	[1]	294,087	[2]
Commercial Real Estate Loans [Member]
Allowance for Loan Losses:
Individually Evaluated for Impairment	3,894		4,368
Collectively Evaluated for Impairment	5,017		4,852
Acquired with Deteriorated Credit Quality	20		77
Total Ending Allowance Balance	8,931		9,297		7,497
Loans:
Loans Individually Evaluated for Impairment	7,550		13,046
Loans Collectively Evaluated for Impairment	473,209		427,063
Loans Acquired with Deteriorated Credit Quality	9,037		13,209
Total Ending Loans Balance (1)	489,796	[1]	453,318	[2]
Agricultural Loans [Member]
Allowance for Loan Losses:
Individually Evaluated for Impairment	150		0
Collectively Evaluated for Impairment	839		926
Acquired with Deteriorated Credit Quality	0		0
Total Ending Allowance Balance	989		926		750
Loans:
Loans Individually Evaluated for Impairment	2,423		0
Loans Collectively Evaluated for Impairment	180,152		170,513
Loans Acquired with Deteriorated Credit Quality	0		0
Total Ending Loans Balance (1)	182,575	[1]	170,513	[2]
Home Equity Loans [Member]
Allowance for Loan Losses:
Individually Evaluated for Impairment	0		0
Collectively Evaluated for Impairment	141		258
Acquired with Deteriorated Credit Quality	0		0
Total Ending Allowance Balance	141		258		220
Loans:
Loans Individually Evaluated for Impairment	0		0
Loans Collectively Evaluated for Impairment	74,699		77,323
Loans Acquired with Deteriorated Credit Quality	0		0
Total Ending Loans Balance (1)	74,699	[1]	77,323	[2]
Consumer Loans [Member]
Allowance for Loan Losses:
Individually Evaluated for Impairment	0		0
Collectively Evaluated for Impairment	214		190
Acquired with Deteriorated Credit Quality	0		0
Total Ending Allowance Balance	214		190		362
Loans:
Loans Individually Evaluated for Impairment	0		0
Loans Collectively Evaluated for Impairment	41,083		47,431
Loans Acquired with Deteriorated Credit Quality	148		164
Total Ending Loans Balance (1)	41,231	[1]	47,595	[2]
Residential Mortgage Loans [Member]
Allowance for Loan Losses:
Individually Evaluated for Impairment	0		0
Collectively Evaluated for Impairment	186		402
Acquired with Deteriorated Credit Quality	0		0
Total Ending Allowance Balance	186		402		543
Loans:
Loans Individually Evaluated for Impairment	0		0
Loans Collectively Evaluated for Impairment	88,666		86,317
Loans Acquired with Deteriorated Credit Quality	149		152
Total Ending Loans Balance (1)	88,815	[1]	86,469	[2]
Unallocated [Member]
Allowance for Loan Losses:
Individually Evaluated for Impairment	0		0
Collectively Evaluated for Impairment	504		746
Acquired with Deteriorated Credit Quality	0		0
Total Ending Allowance Balance	504		746		232
Loans:
Loans Individually Evaluated for Impairment	0		0
Loans Collectively Evaluated for Impairment	0		0
Loans Acquired with Deteriorated Credit Quality	0		0
Total Ending Loans Balance (1)	$ 0		$ 0

[1] Total recorded investment in loans includes $5,522 in accrued interest.

[2] Total recorded investment in loans includes $5,756 in accrued interest.