Loans and Allowance for Loan Losses - Schedule of Loans Modified and Considered Troubled Debt Restructuring (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013 SecurityLoan	Mar. 31, 2012 SecurityLoan
Troubled Debt Restructured Loan Modifications by Class [Line Items]
Number	7	19
Pre-Modification Recorded Investment	$ 940	$ 4,038
Post-Modification Recorded Investment	940	4,038
Commercial [Member]
Troubled Debt Restructured Loan Modifications by Class [Line Items]
Number	2	4
Pre-Modification Recorded Investment	120	127
Post-Modification Recorded Investment	120	127
Agricultural [Member]
Troubled Debt Restructured Loan Modifications by Class [Line Items]
Number	1	2
Pre-Modification Recorded Investment	24	108
Post-Modification Recorded Investment	24	108
Real Estate [Member]
Troubled Debt Restructured Loan Modifications by Class [Line Items]
Number	4	13
Pre-Modification Recorded Investment	796	3,803
Post-Modification Recorded Investment	796	3,803
Consumer [Member]
Troubled Debt Restructured Loan Modifications by Class [Line Items]
Number
Pre-Modification Recorded Investment
Post-Modification Recorded Investment