Loans and Allowance for Loan Losses - Changes in Allowance for Loan Losses (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	$ 33,900	$ 34,839
Provision for loan losses	1,690	401
Recoveries	400	255
Charge-offs	(1,297)	(823)
Ending balance	34,693	34,672
Commercial [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	6,440	7,343
Provision for loan losses	335	(672)
Recoveries	88	121
Charge-offs	(218)	(288)
Ending balance	6,645	6,504
Agricultural [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	383	1,541
Provision for loan losses	(99)	(86)
Recoveries	3	11
Charge-offs	(2)
Ending balance	285	1,466
Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	24,940	24,063
Provision for loan losses	1,233	1,120
Recoveries	187	28
Charge-offs	(731)	(350)
Ending balance	25,629	24,861
Consumer [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	2,137	1,892
Provision for loan losses	221	39
Recoveries	122	95
Charge-offs	(346)	(185)
Ending balance	$ 2,134	$ 1,841