Loans and Allowance for Loan Losses - Loans Held-for-Investment by Class of Financing Receivables (Detail) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015	Sep. 30, 2015
Loans and Leases Receivable Disclosure [Line Items]
Total	$ 3,337,793	$ 3,317,050	$ 3,266,817
Commercial [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total	663,581	696,163	698,406
Agriculture [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total	84,716	102,351	99,232
Real Estate [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total	2,191,260	2,136,233	2,088,002
Consumer [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total	$ 398,236	$ 382,303	$ 381,177