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, 2015	Dec. 31, 2014	Sep. 30, 2014
Financing Receivable, Impaired [Line Items]
Loans individually evaluated for impairment	$ 21,788	$ 20,194	$ 22,093
Loans collectively evaluated for impairment	3,245,029	2,908,994	2,806,337
Total	3,266,817	2,929,188	2,828,430
Commercial [Member]
Financing Receivable, Impaired [Line Items]
Loans individually evaluated for impairment	5,077	3,193	3,481
Loans collectively evaluated for impairment	693,329	636,761	608,611
Total	698,406	639,954	612,092
Agriculture [Member]
Financing Receivable, Impaired [Line Items]
Loans individually evaluated for impairment	75	165	102
Loans collectively evaluated for impairment	99,157	105,529	86,616
Total	99,232	105,694	86,718
Real Estate [Member]
Financing Receivable, Impaired [Line Items]
Loans individually evaluated for impairment	16,124	16,218	17,712
Loans collectively evaluated for impairment	2,071,878	1,806,636	1,756,927
Total	2,088,002	1,822,854	1,774,639
Consumer [Member]
Financing Receivable, Impaired [Line Items]
Loans individually evaluated for impairment	512	618	798
Loans collectively evaluated for impairment	380,665	360,068	354,183
Total	$ 381,177	$ 360,686	$ 354,981