Loans Held for Investment and Allowance for Loan Losses - Schedule of Non-Accrual Loans (Detail) - USD ($) $ in Thousands	Jul. 30, 2019	Jun. 30, 2019		Dec. 31, 2018		Jun. 30, 2018
Financing Receivable, Recorded Investment, Past Due [Line Items]
Non-accrual loans	$ 26,408	$ 26,408	[1]	$ 27,534	[1]	$ 26,685	[1]
Commercial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Non-accrual loans		8,189		9,334		7,580
Agriculture [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Non-accrual loans		1,047		759		1,259
Real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Non-accrual loans		16,787		16,714		16,715
Consumer [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Non-accrual loans		$ 385		$ 727		$ 1,131

[1] Includes $464,000, $2,813,000 and $827,000 of purchased credit impaired loans as of June 30, 2019 and 2018, and December 31, 2018, respectively.