Loans and Allowance for Loan Losses - Schedule of Loans Modified and Considered Troubled Debt Restructuring (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012 SecurityLoan	Dec. 31, 2011 SecurityLoan
Troubled Debt Restructured Loan Modifications by Class [Line Items]
Number	59	21
Pre-Modification Recorded Investment	$ 13,858	$ 7,325
Post-Modification Recorded Investment	13,858	7,325
Commercial [Member]
Troubled Debt Restructured Loan Modifications by Class [Line Items]
Number	18	8
Pre-Modification Recorded Investment	1,180	2,467
Post-Modification Recorded Investment	1,180	2,467
Agricultural [Member]
Troubled Debt Restructured Loan Modifications by Class [Line Items]
Number	5	4
Pre-Modification Recorded Investment	354	2,566
Post-Modification Recorded Investment	354	2,566
Real Estate [Member]
Troubled Debt Restructured Loan Modifications by Class [Line Items]
Number	34	9
Pre-Modification Recorded Investment	12,304	2,292
Post-Modification Recorded Investment	12,304	2,292
Consumer [Member]
Troubled Debt Restructured Loan Modifications by Class [Line Items]
Number	2
Pre-Modification Recorded Investment	20
Post-Modification Recorded Investment	$ 20