Loans Held for Investment and Allowance for Loan Losses - Schedule of Allowance for Loan Losses by Portfolio Segment (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for impairment	$ 4,823		$ 3,996	$ 4,543
Loans collectively evaluated for impairment	45,128		44,160	42,867
Total allowance for loan losses	49,951	$ 49,499	48,156	47,410	$ 46,192	$ 45,779
Commercial [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for impairment	1,379		1,194	1,683
Loans collectively evaluated for impairment	7,839		9,671	10,252
Total allowance for loan losses	9,218	9,277	10,865	11,935	11,682	11,707
Agriculture [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for impairment	368		31	17
Loans collectively evaluated for impairment	1,034		1,274	1,110
Total allowance for loan losses	1,402	1,512	1,305	1,127	946	1,101
Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for impairment	2,645		2,316	2,369
Loans collectively evaluated for impairment	30,598		27,580	25,654
Total allowance for loan losses	33,243	32,539	29,896	28,023	27,279	26,864
Consumer [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for impairment	431		455	474
Loans collectively evaluated for impairment	5,657		5,635	5,851
Total allowance for loan losses	$ 6,088	$ 6,171	$ 6,090	$ 6,325	$ 6,285	$ 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.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
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-SubTopic 210
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