Loans and Allowance for Loan Losses - Changes in Allowance for Loan Losses (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	$ 34,839	$ 34,315
Provision for loan losses	401	1,296
Recoveries	255	323
Charge-offs	(823)	(1,405)
Ending balance	34,672	34,529
Commercial [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	7,343	9,664
Provision for loan losses	(672)	94
Recoveries	121	103
Charge-offs	(288)	(249)
Ending balance	6,504	9,612
Agricultural [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	1,541	1,482
Provision for loan losses	(86)	(119)
Recoveries	11	12
Charge-offs		(22)
Ending balance	1,466	1,353
Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	24,063	21,533
Provision for loan losses	1,120	1,161
Recoveries	28	113
Charge-offs	(350)	(972)
Ending balance	24,861	21,835
Consumer [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	1,892	1,636
Provision for loan losses	39	160
Recoveries	95	95
Charge-offs	(185)	(162)
Ending balance	$ 1,841	$ 1,729