Loans and Allowance for Loan Losses - Loans Held-for-Investment by Class of Financing Receivables (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016
Loans and Leases Receivable Disclosure [Line Items]
Total	$ 3,472,227	$ 3,357,307	$ 3,337,793
Commercial [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total	674,947	674,410	663,581
Agriculture [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total	83,005	84,021	84,716
Real Estate [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total	2,297,556	2,189,844	2,191,260
Consumer [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total	$ 416,719	$ 409,032	$ 398,236