Loans and Allowance for Loan Losses - Schedule of How Loans were Modified as Troubled Debt Restructured Loans (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
Troubled Debt Restructured Loan Modifications by Class [Line Items]
Adjusted Interest Rate	$ 1,687	$ 2,842
Extended Maturity	3,225	4,152
Combined Rate and Maturity	8,946	331
Commercial [Member]
Troubled Debt Restructured Loan Modifications by Class [Line Items]
Adjusted Interest Rate	509	2,350
Extended Maturity	298	118
Combined Rate and Maturity	373
Agricultural [Member]
Troubled Debt Restructured Loan Modifications by Class [Line Items]
Adjusted Interest Rate	243
Extended Maturity	15	2,566
Combined Rate and Maturity	95
Real Estate [Member]
Troubled Debt Restructured Loan Modifications by Class [Line Items]
Adjusted Interest Rate	935	492
Extended Maturity	2,893	1,468
Combined Rate and Maturity	8,477	331
Consumer [Member]
Troubled Debt Restructured Loan Modifications by Class [Line Items]
Adjusted Interest Rate
Extended Maturity	19
Combined Rate and Maturity	$ 1