Loans and Allowance for Loan Losses - Schedule of Investment in Loans Related to Balance in Allowance for Loan Losses on Basis of Company's Impairment Methodology (Detail) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015	Sep. 30, 2015
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for impairment	$ 33,712	$ 28,601	$ 21,788
Loans collectively evaluated for impairment	3,304,081	3,288,449	3,245,029
Total	3,337,793	3,317,050	3,266,817
Commercial [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for impairment	12,714	8,761	5,077
Loans collectively evaluated for impairment	650,867	687,402	693,329
Total	663,581	696,163	698,406
Agriculture [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for impairment	167	97	75
Loans collectively evaluated for impairment	84,549	102,254	99,157
Total	84,716	102,351	99,232
Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for impairment	19,582	18,766	16,124
Loans collectively evaluated for impairment	2,171,678	2,117,467	2,071,878
Total	2,191,260	2,136,233	2,088,002
Consumer [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for impairment	1,249	977	512
Loans collectively evaluated for impairment	396,987	381,326	380,665
Total	$ 398,236	$ 382,303	$ 381,177