Loans and Allowance for Loan Losses - Changes in Allowance for Loan Losses (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	$ 34,693	$ 34,672	$ 33,900	$ 34,839
Provision for loan losses	1,124	832	2,814	1,233
Recoveries	376	165	776	422
Charge-offs	(301)	(1,570)	(1,598)	(2,395)
Ending balance	35,892	34,099	35,892	34,099
Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	25,629	24,861	24,940	24,063
Provision for loan losses	835	271	2,068	1,391
Recoveries	132	23	319	53
Charge-offs	(35)	(1,253)	(766)	(1,605)
Ending balance	26,561	23,902	26,561	23,902
Agricultural [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	285	1,466	383	1,541
Provision for loan losses	78	29	(21)	(57)
Recoveries	1	8	3	19
Charge-offs		(10)	(1)	(10)
Ending balance	364	1,493	364	1,493
Commercial [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	6,645	6,504	6,440	7,343
Provision for loan losses	219	240	554	(431)
Recoveries	65	68	154	188
Charge-offs	(68)	(76)	(287)	(364)
Ending balance	6,861	6,736	6,861	6,736
Consumer [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	2,134	1,841	2,137	1,892
Provision for loan losses	(8)	292	213	330
Recoveries	178	66	300	162
Charge-offs	(198)	(231)	(544)	(416)
Ending balance	$ 2,106	$ 1,968	$ 2,106	$ 1,968