Loans Held for Investment and Allowance for Credit Losses on Loans - Collateral-dependent Gross Loans Held for Investment by Collateral Type and Related Specific Allocation (Detail) - USD ($) $ in Thousands	Jun. 30, 2023	Mar. 31, 2023	Dec. 31, 2022	Jun. 30, 2022	Mar. 31, 2022	Dec. 31, 2021
Financing Receivable Allowance For Credit Losses [Line Items]
Collateral Dependent Loans	$ 15,729		$ 18,171
Specific Allocation	96,920	$ 94,760	92,728	$ 86,935	$ 87,239	$ 83,936
Real Estate Asset [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Collateral Dependent Loans	7,017		7,833
Business Assets [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Collateral Dependent Loans	5,551		6,625
Other Assets [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Collateral Dependent Loans	3,161		3,713
Specific Allocation [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Specific Allocation	5,812		6,597
Real Estate [Member] | Commercial Real Estate Owner Occupied [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Collateral Dependent Loans	2,129		2,213
Specific Allocation	6,808	6,547	6,416	6,928	8,262	7,550
Real Estate [Member] | Commercial Real Estate Non-Owner Occupied [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Collateral Dependent Loans	664		1,263
Specific Allocation	33,432	32,120	30,190	16,315	16,348	16,807
Real Estate [Member] | Construction Residential Real Estate Less than 60 Months [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Collateral Dependent Loans	376		420
Specific Allocation	3,553	3,226	3,275	1,183	1,103	1,051
Real Estate [Member] | Residential Real Estate First Lien [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Collateral Dependent Loans	371		481
Specific Allocation	4,755	4,454	4,092	3,355	3,394	3,048
Real Estate [Member] | Residential Real Estate All Other [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Collateral Dependent Loans	41		9
Specific Allocation	1,661	1,444	1,418	1,480	1,560	1,567
Real Estate [Member] | Agriculture [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Collateral Dependent Loans	7,176		7,740
Real Estate [Member] | Real Estate Asset [Member] | Commercial Real Estate Owner Occupied [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Collateral Dependent Loans	2,129		2,213
Real Estate [Member] | Real Estate Asset [Member] | Commercial Real Estate Non-Owner Occupied [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Collateral Dependent Loans	664		1,263
Real Estate [Member] | Real Estate Asset [Member] | Construction Residential Real Estate Less than 60 Months [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Collateral Dependent Loans	376		420
Real Estate [Member] | Real Estate Asset [Member] | Residential Real Estate First Lien [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Collateral Dependent Loans	371		481
Real Estate [Member] | Real Estate Asset [Member] | Residential Real Estate All Other [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Collateral Dependent Loans	41		9
Real Estate [Member] | Real Estate Asset [Member] | Agriculture [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Collateral Dependent Loans	3,436		3,447
Real Estate [Member] | Business Assets [Member] | Agriculture [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Collateral Dependent Loans	672		701
Real Estate [Member] | Other Assets [Member] | Agriculture [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Collateral Dependent Loans	3,068		3,592
Real Estate [Member] | Specific Allocation [Member] | Commercial Real Estate Owner Occupied [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Specific Allocation	863		870
Real Estate [Member] | Specific Allocation [Member] | Commercial Real Estate Non-Owner Occupied [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Specific Allocation	270		333
Real Estate [Member] | Specific Allocation [Member] | Construction Residential Real Estate Less than 60 Months [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Specific Allocation	45		45
Real Estate [Member] | Specific Allocation [Member] | Residential Real Estate First Lien [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Specific Allocation	118		207
Real Estate [Member] | Specific Allocation [Member] | Residential Real Estate All Other [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Specific Allocation	19		9
Real Estate [Member] | Specific Allocation [Member] | Agriculture [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Specific Allocation	2,694		3,114
Commercial and Agricultural Non-Real Estate [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Collateral Dependent Loans			5,928
Commercial and Agricultural Non-Real Estate [Member] | Business Assets [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Collateral Dependent Loans			5,924
Commercial and Agricultural Non-Real Estate [Member] | Other Assets [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Collateral Dependent Loans			4
Commercial and Agricultural Non-Real Estate [Member] | Specific Allocation [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Specific Allocation			1,938
Commercial Non-real Estate [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Collateral Dependent Loans	4,879
Specific Allocation	25,127	25,079	25,106	25,370	25,163	25,565
Commercial Non-real Estate [Member] | Business Assets [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Collateral Dependent Loans	4,879
Commercial Non-real Estate [Member] | Specific Allocation [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Specific Allocation	1,747
Consumer Non-real Estate [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Collateral Dependent Loans	93		117
Specific Allocation	4,344	$ 4,232	4,132	$ 4,050	$ 3,730	$ 3,694
Consumer Non-real Estate [Member] | Other Assets [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Collateral Dependent Loans	93		117
Consumer Non-real Estate [Member] | Specific Allocation [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Specific Allocation	$ 56		$ 81