Loans and Allowance for Loan Losses - Changes in Allowance for Loan Losses (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	$ 36,824	$ 33,900
PROVISION FOR LOAN LOSSES	1,290	1,690
Recoveries	224	400
Charge-offs	(510)	(1,297)
Ending balance	37,828	34,693
Commercial [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	7,990	6,440
PROVISION FOR LOAN LOSSES	2,465	335
Recoveries	80	88
Charge-offs	(160)	(218)
Ending balance	10,375	6,645
Agriculture [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	527	383
PROVISION FOR LOAN LOSSES	(32)	(99)
Recoveries	2	3
Charge-offs	(24)	(2)
Ending balance	473	285
Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	26,657	24,940
PROVISION FOR LOAN LOSSES	(1,467)	1,233
Recoveries	71	187
Charge-offs	(22)	(731)
Ending balance	25,239	25,629
Consumer [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	1,650	2,137
PROVISION FOR LOAN LOSSES	324	221
Recoveries	71	122
Charge-offs	(304)	(346)
Ending balance	$ 1,741	$ 2,134