Loans and Allowance for Loan Losses - Schedule of Non-Accrual Loans (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Financing Receivable, Recorded Investment, Past Due [Line Items]
Non-accrual loans	$ 21,800	$ 19,975
Commercial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Non-accrual loans	2,251	3,450
Agricultural [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Non-accrual loans	372	145
Real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Non-accrual loans	18,698	16,193
Consumer [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Non-accrual loans	$ 479	$ 187

Reflects the carrying amount of loans deemed to be questionable as to collection on which no interest is continuing to be recognized.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 50

-Paragraph 7

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=7512638&loc=d3e5111-111524

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

-Publisher AICPA

-Name Statement of Position (SOP)

-Number 01-6

-Paragraph 13

-Subparagraph g

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.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.

Name:	us-gaap_LoansAndLeasesReceivableImpairedNonperformingNonaccrualOfInterest
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant