Loans Held for Investment and Allowance for Loan Losses - Schedule of Non-Accrual Loans (Detail) - USD ($) $ in Thousands		Jun. 30, 2020	Dec. 31, 2019	Jun. 30, 2019
Financing Receivable, Recorded Investment, Past Due [Line Items]
Non-accrual loans	[1]	$ 39,320	$ 24,582	$ 26,408
Commercial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Non-accrual loans		4,618	3,093	8,189
Agriculture [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Non-accrual loans		1,056	1,376	1,047
Real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Non-accrual loans		33,170	19,787	16,787
Consumer [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Non-accrual loans		$ 476	$ 326	$ 385

[1] Includes $7,275,000, $464,000 and $251,000 of purchased credit impaired loans as of June 30, 2020 and 2019, and December 31, 2019, respectively.