Loans Held for Investment and Allowance for Loan Losses - Schedule of Allowance for Loan Losses by Portfolio Segment (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for impairment	$ 4,069	$ 3,996
Loan collectively evaluated for impairment	47,133	44,160
Total allowance for loan losses	51,202	48,156	$ 45,779
Commercial [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for impairment	1,133	1,194
Loan collectively evaluated for impairment	10,815	9,671
Total allowance for loan losses	11,948	10,865	11,707
Agriculture [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for impairment	170	31
Loan collectively evaluated for impairment	1,276	1,274
Total allowance for loan losses	1,446	1,305	1,101
Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for impairment	2,409	2,316
Loan collectively evaluated for impairment	29,933	27,580
Total allowance for loan losses	32,342	29,896	26,864
Consumer [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for impairment	357	455
Loan collectively evaluated for impairment	5,109	5,635
Total allowance for loan losses	$ 5,466	$ 6,090	$ 6,107

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
-Topic 310
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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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