Loans and Allowance for Loan Losses - Loans Held-for-Investment by Class of Financing Receivables (Detail) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016	Jun. 30, 2016
Loans and Leases Receivable Disclosure [Line Items]
Total	$ 3,439,352	$ 3,357,307	$ 3,283,655
Commercial [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total	668,049	674,410	661,659
Agriculture [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total	77,342	84,021	80,812
Real Estate [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total	2,271,100	2,189,844	2,154,388
Consumer [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total	$ 422,861	$ 409,032	$ 386,796