Loans Held for Investment and Allowance for Loan Losses - Schedule of Allowance for Loan Losses by Portfolio Segment (Detail) - USD ($) $ in Thousands	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for impairment	$ 3,046		$ 3,228	$ 3,866
Loan collectively evaluated for impairment	65,901		49,271	47,954
Total allowance for loan losses	68,947	$ 60,440	52,499	51,820	$ 51,585	$ 51,202
Commercial [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for impairment	1,250		1,042	1,022
Loan collectively evaluated for impairment	17,322		11,080	12,877
Total allowance for loan losses	18,572	11,773	12,122	13,899	12,475	11,948
Agriculture [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for impairment	83		235	119
Loan collectively evaluated for impairment	2,461		971	1,241
Total allowance for loan losses	2,544	2,154	1,206	1,360	1,430	1,446
Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for impairment	1,711		1,950	2,515
Loan collectively evaluated for impairment	40,912		32,024	28,284
Total allowance for loan losses	42,623	41,256	33,974	30,799	31,887	32,342
Consumer [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for impairment	2		1	210
Loan collectively evaluated for impairment	5,206		5,196	5,552
Total allowance for loan losses	$ 5,208	$ 5,257	$ 5,197	$ 5,762	$ 5,793	$ 5,466

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

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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
-Topic 310
-SubTopic 10
-Section 50
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-Publisher FASB
-Name Accounting Standards Codification
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-Name Accounting Standards Codification
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-URI http://asc.fasb.org/extlink&oid=27010918&loc=d3e74567-122707

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