Allowance for Loan Losses by Portfolio Class (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Financing Receivable Allowance For Credit Losses [Line Items]
Beginning Balance	$ 15,312	$ 13,317	$ 11,016
Provision for Loan Losses	2,412	6,800	5,225
Recoveries	326	384	1,290
Loan Charged-off	(2,530)	(5,189)	(4,214)
Ending Balance	15,520	15,312	13,317
Commercial and Industrial Loans and Leases [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Beginning Balance	3,493	3,713
Provision for Loan Losses	1,150	1,195
Recoveries	74	98
Loan Charged-off	(162)	(1,513)
Ending Balance	4,555	3,493
Commercial Real Estate Loans [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Beginning Balance	9,297	7,497
Provision for Loan Losses	1,326	4,265
Recoveries	97	139
Loan Charged-off	(1,789)	(2,604)
Ending Balance	8,931	9,297
Agricultural Loans [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Beginning Balance	926	750
Provision for Loan Losses	63	176
Recoveries	0	0
Loan Charged-off	0	0
Ending Balance	989	926
Home Equity Loans [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Beginning Balance	258	220
Provision for Loan Losses	(32)	287
Recoveries	2	6
Loan Charged-off	(87)	(255)
Ending Balance	141	258
Consumer Loans [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Beginning Balance	190	362
Provision for Loan Losses	194	23
Recoveries	123	125
Loan Charged-off	(293)	(320)
Ending Balance	214	190
Residential Mortgage Loans [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Beginning Balance	402	543
Provision for Loan Losses	(47)	340
Recoveries	30	16
Loan Charged-off	(199)	(497)
Ending Balance	186	402
Unallocated [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Beginning Balance	746	232
Provision for Loan Losses	(242)	514
Recoveries	0	0
Loan Charged-off	0	0
Ending Balance	$ 504	$ 746

Reflects a reduction in bad debt expense attributable to collections of loans and lease receivables which had previously been fully or partially charged off as bad debts.

+ References+ Details

Reflects the amount of loans and leases that have been removed, or charged off, from both loan receivables and the reserve for credit losses, typically because they are considered to be not salvageable or have been purchased by unconsolidated collection entities.

+ References+ Details

- Definition

Amount of allowance to cover probable credit losses on loans and leases. Includes carryover of or adjustments to the allowance for loan losses in connection with business combinations. Excludes allowance for loans and leases covered under loss sharing agreements.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 5

-Paragraph 8, 9

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 30

-Section 50

-Paragraph 2

-Subparagraph (b)(1)(ii)

-URI http://asc.fasb.org/extlink&oid=6379141&loc=d3e15032-111544

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 210

-Section S99

-Paragraph 1

-Subparagraph (SX 210.9-03.7(d))

-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher OTS

-Name Federal Regulation (FR)

-Number Title 12

-Section 563c.102

-Paragraph 8

-Subparagraph a(ii)

-Chapter V

-Subsection I

-LegacyDoc This is a non-GAAP reference that was included in the 2009 taxonomy. It will be removed from future versions of this taxonomy.

Reference 5: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 7

-Subparagraph d

-Article 9

Reference 6: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 114

-Paragraph 20

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 7: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Staff Accounting Bulletin (SAB)

-Number Topic 6

-Section L

-Subsection 1

+ Details

- Definition

Provision charged for the period based on estimated losses to be realized from loan and lease transactions.

+ References+ Details