Loans Held for Investment and Allowance for Loan Losses - Schedule of Internal Ratings of Loans (Detail) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020	Mar. 31, 2020
Financing Receivable, Credit Quality Indicator [Line Items]
Internal ratings of loan	$ 5,322,562	$ 5,171,033	$ 4,639,389
Commercial [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Internal ratings of loan	1,355,075	1,312,707	869,450
Agriculture [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Internal ratings of loan	90,366	94,864	99,582
Real Estate [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Internal ratings of loan	3,414,751	3,319,265	3,249,249
Consumer [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Internal ratings of loan	$ 462,370	$ 444,197	421,108
Pass [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Internal ratings of loan			4,449,000
Pass [Member] | Commercial [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Internal ratings of loan			832,000
Pass [Member] | Agriculture [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Internal ratings of loan			92,000
Pass [Member] | Real Estate [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Internal ratings of loan			3,106,000
Pass [Member] | Consumer [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Internal ratings of loan			419,000
Special Mention [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Internal ratings of loan			87,000
Special Mention [Member] | Commercial [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Internal ratings of loan			25,000
Special Mention [Member] | Agriculture [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Internal ratings of loan			6,000
Special Mention [Member] | Real Estate [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Internal ratings of loan			56,000
Substandard [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Internal ratings of loan			103,000
Substandard [Member] | Commercial [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Internal ratings of loan			12,000
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			87,000
Substandard [Member] | Consumer [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Internal ratings of loan			$ 2,000