Loans Held for Investment and Allowance for Loan Losses - Schedule of Internal Ratings of Loans (Detail) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020	Jun. 30, 2020
Financing Receivable, Credit Quality Indicator [Line Items]
Internal ratings of loan	$ 5,304,602	$ 5,171,033	$ 5,253,067
Commercial [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Internal ratings of loan	1,162,459	1,312,707	1,509,454
Agriculture [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Internal ratings of loan	95,212	94,864	97,448
Internal ratings of loan			98,000
Real Estate [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Internal ratings of loan	3,558,353	3,319,265	3,235,208
Consumer [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Internal ratings of loan	$ 488,578	$ 444,197	410,957
Pass [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Internal ratings of loan			5,052,000
Pass [Member] | Commercial [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Internal ratings of loan			1,465,000
Pass [Member] | Agriculture [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Internal ratings of loan			91,000
Pass [Member] | Real Estate [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Internal ratings of loan			3,087,000
Pass [Member] | Consumer [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Internal ratings of loan			409,000
Special Mention [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Internal ratings of loan			64,000
Special Mention [Member] | Commercial [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Internal ratings of loan			9,000
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			50,000
Substandard [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Internal ratings of loan			137,000
Substandard [Member] | Commercial [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Internal ratings of loan			35,000
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			98,000
Substandard [Member] | Consumer [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Internal ratings of loan			$ 2,000