Loans Held for Investment and Allowance for Credit Losses - Loans Held-for-Investment by Class of Financing Receivables (Detail) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2019
Loans and Leases Receivable Disclosure [Line Items]
Total	$ 4,639,389	$ 4,194,969	$ 3,989,160
Commercial [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total	869,450	856,326	826,886
Agriculture [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total	99,582	103,640	91,336
Real Estate [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total	3,249,249	2,823,372	2,684,207
Consumer [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total	$ 421,108	$ 411,631	$ 386,731