Loans and Allowance for Loan Losses - Loans Held-for-Investment by Class of Financing Receivables (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017
Loans and Leases Receivable Disclosure [Line Items]
Total	$ 3,730,051	$ 3,485,569	$ 3,372,512
Commercial [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total	734,156	684,099	666,687
Agriculture [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total	95,958	94,543	79,237
Real Estate [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total	2,502,904	2,302,998	2,209,021
Consumer [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total	$ 397,033	$ 403,929	$ 417,567