Loans and Allowance for Loan Losses - Schedule of Allowance for Loan Loss by Portfolio Segment (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for impairment	$ 12,329	$ 13,997
Loans collectively evaluated for impairment	22,510	20,318
Total allowance for loan losses	34,839	34,315	31,106
Commercial [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for impairment	3,253	4,647
Loans collectively evaluated for impairment	4,090	5,017
Total allowance for loan losses	7,343	9,664	7,745
Agricultural [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for impairment	388	758
Loans collectively evaluated for impairment	1,153	724
Total allowance for loan losses	1,541	1,482	2,299
Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for impairment	8,380	8,310
Loans collectively evaluated for impairment	15,683	13,223
Total allowance for loan losses	24,063	21,533	19,101
Consumer [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for impairment	308	282
Loans collectively evaluated for impairment	1,584	1,354
Total allowance for loan losses	$ 1,892	$ 1,636	$ 1,961