Loans Held for Investment and Allowance for Loan Losses - Loans Held-for-Investment by Class of Financing Receivables (Detail) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019	Jun. 30, 2019
Loans and Leases Receivable Disclosure [Line Items]
Total loans held-for-investment	$ 5,253,067	$ 4,194,969	$ 4,040,952
Commercial [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans held-for-investment	1,509,454	856,326	813,887
Agriculture [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans held-for-investment	97,448	103,640	97,535
Real Estate [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans held-for-investment	3,235,208	2,823,372	2,730,585
Consumer [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans held-for-investment	$ 410,957	$ 411,631	$ 398,945