Loans Held for Investment and Allowance for Credit Losses - Schedule of Allowance for Loan Losses by Portfolio Segment (Detail) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2019	Dec. 31, 2018
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for impairment	$ 3,386	$ 3,228	$ 4,472
Loan collectively evaluated for impairment	57,054	49,271	47,113
Total allowance for loan losses	60,440	52,499	51,585	$ 51,202
Commercial [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for impairment	1,271	1,042	1,184
Loan collectively evaluated for impairment	10,502	11,080	11,291
Total allowance for loan losses	11,773	12,122	12,475	11,948
Agriculture [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for impairment	98	235	130
Loan collectively evaluated for impairment	2,056	971	1,300
Total allowance for loan losses	2,154	1,206	1,430	1,446
Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for impairment	2,016	1,950	2,901
Loan collectively evaluated for impairment	39,240	32,024	28,986
Total allowance for loan losses	41,256	33,974	31,887	32,342
Consumer [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for impairment	1	1	257
Loan collectively evaluated for impairment	5,256	5,196	5,536
Total allowance for loan losses	$ 5,257	$ 5,197	$ 5,793	$ 5,466

The valuation allowance for financing receivables that are expected to be uncollectible that were collectively evaluated for impairment.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
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-Section 50
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Amount of valuation allowance for financing receivables that are expected to be uncollectible that were individually evaluated for impairment.

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Amount of allowance to cover probable credit losses on loans and leases. Includes carryover of or adjustments to the allowance for loan losses in connection with business combinations. Excludes allowance for loans and leases covered under loss sharing agreements.

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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
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