Loans and Allowance for Loan Losses - Changes in Allowance for Loan Losses (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	$ 47,410	$ 45,060	$ 45,779	$ 41,877
Provision for loan losses	1,415	3,833	5,090	8,219
Recoveries	277	777	1,434	3,243
Charge-offs	(1,180)	(4,372)	(4,381)	(8,041)
Ending balance	47,922	45,298	47,922	45,298
Commercial [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	11,935	14,026	11,707	12,644
Provision for loan losses	557	3,248	1,485	6,239
Recoveries	119	298	868	839
Charge-offs	(739)	(3,856)	(2,188)	(6,006)
Ending balance	11,872	13,716	11,872	13,716
Agriculture [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	1,127	1,451	1,101	1,191
Provision for loan losses	157	(358)	211	41
Recoveries	17	4	25	20
Charge-offs		(29)	(36)	(184)
Ending balance	1,301	1,068	1,301	1,068
Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	28,023	25,644	26,864	24,375
Provision for loan losses	424	296	2,556	367
Recoveries	50	367	141	1,957
Charge-offs	(29)	(86)	(1,093)	(478)
Ending balance	28,468	26,221	28,468	26,221
Consumer [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	6,325	3,939	6,107	3,667
Provision for loan losses	277	647	838	1,572
Recoveries	91	108	400	427
Charge-offs	(412)	(401)	(1,064)	(1,373)
Ending balance	$ 6,281	$ 4,293	$ 6,281	$ 4,293