Loans and Allowance for Loan Losses - Schedule of Non-Accrual Loans (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Financing Receivable, Recorded Investment, Past Due [Line Items]
Non-accrual loans	$ 20,194	$ 27,926
Agriculture [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Non-accrual loans	165	131
Real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Non-accrual loans	16,218	22,548
Commercial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Non-accrual loans	3,193	4,281
Consumer [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Non-accrual loans	$ 618	$ 966