Loans and Allowance for Loan Losses - Loans Held-for-Investment by Class of Financing Receivables (Detail) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016	Mar. 31, 2016
Loans and Leases Receivable Disclosure [Line Items]
Total	$ 3,372,512	$ 3,357,307	$ 3,282,199
Commercial [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total	666,687	674,410	669,525
Agriculture [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total	79,237	84,021	87,490
Real Estate [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total	2,209,021	2,189,844	2,150,132
Consumer [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total	$ 417,567	$ 409,032	$ 375,052