Loans Held for Investment and Allowance for Loan Losses - Schedule of Allowance for Loan Losses by Portfolio Segment (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for impairment	$ 3,866		$ 4,069	$ 4,823
Loans collectively evaluated for impairment	47,954		47,133	45,128
Total allowance for loan losses	51,820	$ 51,585	51,202	49,951	$ 49,499	$ 48,156
Commercial [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for impairment	1,022		1,133	1,379
Loans collectively evaluated for impairment	12,877		10,815	7,839
Total allowance for loan losses	13,899	12,475	11,948	9,218	9,277	10,865
Agriculture [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for impairment	119		170	368
Loans collectively evaluated for impairment	1,241		1,276	1,034
Total allowance for loan losses	1,360	1,430	1,446	1,402	1,512	1,305
Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for impairment	2,515		2,409	2,645
Loans collectively evaluated for impairment	28,284		29,933	30,598
Total allowance for loan losses	30,799	31,887	32,342	33,243	32,539	29,896
Consumer [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for impairment	210		357	431
Loans collectively evaluated for impairment	5,552		5,109	5,657
Total allowance for loan losses	$ 5,762	$ 5,793	$ 5,466	$ 6,088	$ 6,171	$ 6,090

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
-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
-Name Accounting Standards Codification
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-SubTopic 210
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