Loans Held for Investment and Allowance for Loan Losses - Schedule of Internal Ratings of Loans (Detail) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020	Sep. 30, 2020
Financing Receivable, Credit Quality Indicator [Line Items]
Internal ratings of loan	$ 5,286,494	$ 5,171,033	$ 5,293,679
Commercial [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Internal ratings of loan	985,444	1,312,707	1,488,345
Agriculture [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Internal ratings of loan	98,947	94,864	93,972
Internal ratings of loan			94,000
Real Estate [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Internal ratings of loan	3,687,581	3,319,265	3,287,605
Consumer [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Internal ratings of loan	$ 514,522	$ 444,197	423,757
Pass [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Internal ratings of loan			5,092,000
Pass [Member] | Commercial [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Internal ratings of loan			1,440,000
Pass [Member] | Agriculture [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Internal ratings of loan			87,000
Pass [Member] | Real Estate [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Internal ratings of loan			3,143,000
Pass [Member] | Consumer [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Internal ratings of loan			422,000
Special Mention [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Internal ratings of loan			66,000
Special Mention [Member] | Commercial [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Internal ratings of loan			16,000
Special Mention [Member] | Agriculture [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Internal ratings of loan			5,000
Special Mention [Member] | Real Estate [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Internal ratings of loan			45,000
Substandard [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Internal ratings of loan			136,000
Substandard [Member] | Commercial [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Internal ratings of loan			32,000
Substandard [Member] | Agriculture [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Internal ratings of loan			2,000
Substandard [Member] | Real Estate [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Internal ratings of loan			100,000
Substandard [Member] | Consumer [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Internal ratings of loan			$ 2,000