Loans Held for Investment and Allowance for Loan Losses - Schedule of Internal Ratings of Loans (Detail) - USD ($) $ in Thousands	Sep. 30, 2022	Dec. 31, 2021	Sep. 30, 2021
Financing Receivable, Credit Quality Indicator [Line Items]
Internal ratings of loan	$ 6,255,488	$ 5,388,972	$ 5,286,494
Commercial [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Internal ratings of loan	1,086,187	1,014,980	985,444
Agriculture [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Internal ratings of loan	76,937	98,089	98,947
Real Estate [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Internal ratings of loan	4,405,773	3,746,519	3,687,581
Consumer [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Internal ratings of loan	$ 686,591	$ 529,384	$ 514,522