Loans Held for Investment and Allowance for Loan Losses - Schedule of Allowance for Loan Losses by Portfolio Segment (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Jun. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Dec. 31, 2016
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for impairment	$ 4,988		$ 3,996	$ 4,177
Loans collectively evaluated for impairment	45,883		44,160	43,745
Total allowance for loan losses	50,871	$ 49,951	48,156	47,922	$ 47,410	$ 45,779
Commercial [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for impairment	1,412		1,194	1,671
Loans collectively evaluated for impairment	7,549		9,671	10,201
Total allowance for loan losses	8,961	9,218	10,865	11,872	11,935	11,707
Agriculture [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for impairment	528		31	17
Loans collectively evaluated for impairment	1,106		1,274	1,284
Total allowance for loan losses	1,634	1,402	1,305	1,301	1,127	1,101
Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for impairment	2,652		2,316	1,984
Loans collectively evaluated for impairment	31,713		27,580	26,484
Total allowance for loan losses	34,365	33,243	29,896	28,468	28,023	26,864
Consumer [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for impairment	396		455	505
Loans collectively evaluated for impairment	5,515		5,635	5,776
Total allowance for loan losses	$ 5,911	$ 6,088	$ 6,090	$ 6,281	$ 6,325	$ 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
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (g)
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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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-SubTopic 210
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