Loans and Allowance for Loan Losses - Schedule of Investment in Loans Related to Balance in Allowance for Loan Losses on Basis of Company's Impairment Methodology (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for impairment	$ 22,752	$ 17,670	$ 28,080
Loans collectively evaluated for impairment	3,707,299	3,467,899	3,344,432
Total	3,730,051	3,485,569	3,372,512
Commercial [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for impairment	5,351	3,612	6,953
Loans collectively evaluated for impairment	728,805	680,487	659,734
Total	734,156	684,099	666,687
Agriculture [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for impairment	143	134	72
Loans collectively evaluated for impairment	95,815	94,409	79,165
Total	95,958	94,543	79,237
Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for impairment	16,161	12,838	19,763
Loans collectively evaluated for impairment	2,486,743	2,290,160	2,189,258
Total	2,502,904	2,302,998	2,209,021
Consumer [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for impairment	1,097	1,086	1,292
Loans collectively evaluated for impairment	395,936	402,843	416,275
Total	$ 397,033	$ 403,929	$ 417,567

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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- Definition

The balance of financing receivables that were collectively evaluated for impairment.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (h)
-URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953423-111524

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- Definition

The balance of financing receivables that were individually evaluated for impairment.

+ References

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- Definition

Amount before allowance of loans and leases held in portfolio, including but not limited to, commercial and consumer loans. Includes deferred interest and fees, undisbursed portion of loan balance, unamortized costs and premiums and discounts from face amounts. Excludes loans and leases covered under loss sharing agreements.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=82891408&loc=d3e4428-111522

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