Loans and Allowance for Loan Losses - Schedule of Allowance for Loan Losses by Portfolio Segment (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012	Mar. 31, 2012	Dec. 31, 2011
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for impairment	$ 12,228	$ 12,329
Loans collectively evaluated for impairment	22,444	22,510
Total allowance for loan losses	34,672	34,839	34,529	34,315
Commercial [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for impairment	2,542	3,253
Loans collectively evaluated for impairment	3,962	4,090
Total allowance for loan losses	6,504	7,343	9,612	9,664
Agricultural [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for impairment	388	388
Loans collectively evaluated for impairment	1,078	1,153
Total allowance for loan losses	1,466	1,541	1,353	1,482
Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for impairment	8,983	8,380
Loans collectively evaluated for impairment	15,878	15,683
Total allowance for loan losses	24,861	24,063	21,835	21,533
Consumer [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for impairment	315	308
Loans collectively evaluated for impairment	1,526	1,584
Total allowance for loan losses	$ 1,841	$ 1,892	$ 1,729	$ 1,636