Loans Held for Investment and Allowance for Loan Losses - Loans Held-for-Investment by Class of Financing Receivables (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018	Jun. 30, 2018
Loans and Leases Receivable Disclosure [Line Items]
Total	$ 4,040,952	$ 3,953,636	$ 3,831,480
Commercial [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total	813,887	844,953	775,010
Agriculture [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total	97,535	96,677	92,583
Real Estate [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total	2,730,585	2,639,346	2,567,878
Consumer [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total	$ 398,945	$ 372,660	$ 396,009