Loans and Allowance for Loan Losses - Changes in Allowance for Loan Losses (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	$ 45,060	$ 38,999	$ 41,877	$ 36,824
Provision for loan losses	3,833	2,664	8,219	5,508
Recoveries	777	234	3,243	1,018
Charge-offs	(4,372)	(1,477)	(8,041)	(2,930)
Ending balance	45,298	40,420	45,298	40,420
Commercial [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	14,026	11,456	12,644	7,990
Provision for loan losses	3,248	1,283	6,239	5,072
Recoveries	298	52	839	249
Charge-offs	(3,856)	(930)	(6,006)	(1,450)
Ending balance	13,716	11,861	13,716	11,861
Agriculture [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	1,451	392	1,191	527
Provision for loan losses	(358)	544	41	486
Recoveries	4		20	2
Charge-offs	(29)	(22)	(184)	(101)
Ending balance	1,068	914	1,068	914
Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	25,644	24,342	24,375	26,657
Provision for loan losses	296	(369)	367	(2,884)
Recoveries	367	65	1,957	438
Charge-offs	(86)	(240)	(478)	(413)
Ending balance	26,221	23,798	26,221	23,798
Consumer [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	3,939	2,809	3,667	1,650
Provision for loan losses	647	1,206	1,572	2,834
Recoveries	108	117	427	329
Charge-offs	(401)	(285)	(1,373)	(966)
Ending balance	$ 4,293	$ 3,847	$ 4,293	$ 3,847