Loans and Allowance for Loan Losses - Loans Held-for-Investment by Class of Financing Receivables (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Loans and Leases Receivable Disclosure [Line Items]
Total	$ 3,357,307	$ 3,317,050
Commercial [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total	674,410	696,163
Agriculture [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total	84,021	102,351
Real Estate [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total	2,189,844	2,136,233
Consumer [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total	$ 409,032	$ 382,303