Loans and Allowance for Loan Losses - Schedule of How Loans Were Modified as Troubled Debt Restructured Loans (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Troubled Debt Restructured Loan Modifications by Class [Line Items]
Adjusted Interest Rate	$ 272	$ 17
Extended Maturity	494	75
Combined Rate and Maturity	174	3,946
Commercial [Member]
Troubled Debt Restructured Loan Modifications by Class [Line Items]
Adjusted Interest Rate		17
Extended Maturity	120	62
Combined Rate and Maturity		48
Agricultural [Member]
Troubled Debt Restructured Loan Modifications by Class [Line Items]
Adjusted Interest Rate
Extended Maturity	24	13
Combined Rate and Maturity		95
Real Estate [Member]
Troubled Debt Restructured Loan Modifications by Class [Line Items]
Adjusted Interest Rate	272
Extended Maturity	350
Combined Rate and Maturity	174	3,803
Consumer [Member]
Troubled Debt Restructured Loan Modifications by Class [Line Items]
Adjusted Interest Rate
Extended Maturity
Combined Rate and Maturity