Loans and Allowance for Loan Losses - Schedule of Non-Accrual Loans (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012	Mar. 31, 2012
Financing Receivable, Recorded Investment, Past Due [Line Items]
Non-accrual loans	$ 22,509	$ 21,800	$ 20,963
Commercial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Non-accrual loans	1,133	2,251	3,665
Agricultural [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Non-accrual loans	473	372	923
Real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Non-accrual loans	20,502	18,698	16,026
Consumer [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Non-accrual loans	$ 401	$ 479	$ 349