Loans Held for Investment and Allowance for Loan Losses - Loans Held-for-Investment by Class of Financing Receivables (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Loans and Leases Receivable Disclosure [Line Items]
Total	$ 3,953,636	$ 3,485,569
Commercial [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total	844,953	684,099
Agriculture [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total	96,677	94,543
Real Estate [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total	2,639,346	2,302,998
Consumer [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total	$ 372,660	$ 403,929