Loans and Allowance for Loan Losses - Changes in Allowance for Loan Losses (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	$ 38,999	$ 35,892	$ 36,824	$ 33,900
PROVISION FOR LOAN LOSSES	2,664	896	5,508	3,710
Recoveries	234	214	1,018	989
Charge-offs	(1,477)	(614)	(2,930)	(2,211)
Ending balance	40,420	36,388	40,420	36,388
Commercial [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	11,456	6,861	7,990	6,440
PROVISION FOR LOAN LOSSES	1,283	635	5,072	1,189
Recoveries	52	72	249	225
Charge-offs	(930)	(82)	(1,450)	(368)
Ending balance	11,861	7,486	11,861	7,486
Agriculture [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	392	364	527	383
PROVISION FOR LOAN LOSSES	544	7	486	(14)
Recoveries		14	2	17
Charge-offs	(22)		(101)	(1)
Ending balance	914	385	914	385
Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	24,342	26,561	26,657	24,940
PROVISION FOR LOAN LOSSES	(369)	(80)	(2,884)	1,988
Recoveries	65	42	438	361
Charge-offs	(240)	(224)	(413)	(990)
Ending balance	23,798	26,299	23,798	26,299
Consumer [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	2,809	2,106	1,650	2,137
PROVISION FOR LOAN LOSSES	1,206	334	2,834	547
Recoveries	117	86	329	386
Charge-offs	(285)	(308)	(966)	(852)
Ending balance	$ 3,847	$ 2,218	$ 3,847	$ 2,218