Loans and Allowance for Loan Losses - Changes in Allowance for Loan Losses (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	$ 34,315	$ 31,106
Provision (credit) for loan losses	3,484	6,626
Recoveries	1,395	1,907
Charge-offs	(4,355)	(5,324)
Ending balance	34,839	34,315
Commercial [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	9,664	7,745
Provision (credit) for loan losses	(2,103)	1,949
Recoveries	281	610
Charge-offs	(499)	(640)
Ending balance	7,343	9,664
Agricultural [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	1,482	2,299
Provision (credit) for loan losses	58	(755)
Recoveries	54	33
Charge-offs	(53)	(95)
Ending balance	1,541	1,482
Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	21,533	19,101
Provision (credit) for loan losses	4,842	5,240
Recoveries	639	874
Charge-offs	(2,951)	(3,682)
Ending balance	24,063	21,533
Consumer [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	1,636	1,961
Provision (credit) for loan losses	687	192
Recoveries	421	390
Charge-offs	(852)	(907)
Ending balance	$ 1,892	$ 1,636