Loans Held for Investment and Allowance for Credit Losses - Schedule of Non-Accrual Loans (Detail) - USD ($) $ in Thousands		Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2019
Financing Receivable, Recorded Investment, Past Due [Line Items]
Non-accrual loans	[1]	$ 39,226	$ 24,582	$ 28,508
Commercial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Non-accrual loans		5,620	3,093	8,897
Agriculture [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Non-accrual loans		1,128	1,376	831
Real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Non-accrual loans		32,156	19,787	18,254
Consumer [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Non-accrual loans		$ 322	$ 326	$ 526

[1] Includes $7,773,000, $859,000 and $251,000 of purchased credit impaired loans as of March 31, 2020 and 2019, and December 31, 2019, respectively.