Loans and Allowance for Loan Losses - Schedule of Troubled Debt Restructuring (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013 Contract
Troubled Debt Restructured Loan Modifications by Class [Line Items]
Number	11
Balance	$ 1,167
Commercial [Member]
Troubled Debt Restructured Loan Modifications by Class [Line Items]
Number	5
Balance	217
Agricultural [Member]
Troubled Debt Restructured Loan Modifications by Class [Line Items]
Number
Balance
Real Estate [Member]
Troubled Debt Restructured Loan Modifications by Class [Line Items]
Number	5
Balance	931
Consumer [Member]
Troubled Debt Restructured Loan Modifications by Class [Line Items]
Number	1
Balance	$ 19