Loans Held for Investment and Allowance for Loan Losses - Schedule of Internal Ratings of Loans (Detail) - USD ($) $ in Thousands	Mar. 31, 2022	Dec. 31, 2021	Mar. 31, 2021
Financing Receivable, Credit Quality Indicator [Line Items]
Internal ratings of loan	$ 5,566,169	$ 5,388,972	$ 5,322,562
Commercial [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Internal ratings of loan	1,029,848	1,014,980	1,355,075
Agriculture [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Internal ratings of loan	82,883	98,089	90,366
Real Estate [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Internal ratings of loan	3,901,656	3,746,519	3,414,751
Consumer [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Internal ratings of loan	$ 551,782	$ 529,384	$ 462,370