Loans and Allowance for Loan Losses - Schedule of Impaired Loans and Related Allowance (Detail) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015
Financing Receivable, Impaired [Line Items]
Unpaid Contractual Principal Balance	$ 47,473	$ 29,966	$ 35,030
Recorded Investment With No Allowance	7,017	5,097	6,546
Recorded Investment With Allowance	26,695	16,691	22,055
Total Recorded Investment	33,712	21,788	28,601
Related Allowance	7,042	4,645	5,071
Year Average Recorded Investment	26,514	19,755	21,735
Three-month Average Recorded Investment	34,924	20,026
Commercial [Member]
Financing Receivable, Impaired [Line Items]
Unpaid Contractual Principal Balance	21,696	6,351	10,056
Recorded Investment With No Allowance	1,067	413	608
Recorded Investment With Allowance	11,647	4,664	8,153
Total Recorded Investment	12,714	5,077	8,761
Related Allowance	3,983	1,666	2,030
Year Average Recorded Investment	8,421	4,037	5,812
Three-month Average Recorded Investment	14,238	4,316
Agriculture [Member]
Financing Receivable, Impaired [Line Items]
Unpaid Contractual Principal Balance	168	121	97
Recorded Investment With Allowance	167	75	97
Total Recorded Investment	167	75	97
Related Allowance	41	47	70
Year Average Recorded Investment	83	55	48
Three-month Average Recorded Investment	84	85
Real Estate [Member]
Financing Receivable, Impaired [Line Items]
Unpaid Contractual Principal Balance	24,130	22,796	23,710
Recorded Investment With No Allowance	5,626	4,341	5,314
Recorded Investment With Allowance	13,956	11,783	13,452
Total Recorded Investment	19,582	16,124	18,766
Related Allowance	2,566	2,844	2,827
Year Average Recorded Investment	17,021	15,222	15,211
Three-month Average Recorded Investment	19,436	15,114
Consumer [Member]
Financing Receivable, Impaired [Line Items]
Unpaid Contractual Principal Balance	1,479	698	1,167
Recorded Investment With No Allowance	324	343	624
Recorded Investment With Allowance	925	169	353
Total Recorded Investment	1,249	512	977
Related Allowance	452	88	144
Year Average Recorded Investment	989	441	$ 664
Three-month Average Recorded Investment	$ 1,166	$ 511

Average recorded investment in impaired loans.

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Average amount of investment of impaired financing receivables with related allowance for credit losses and without a related allowance for credit losses.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 15
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=66008989&loc=d3e5212-111524

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 17
-URI http://asc.fasb.org/extlink&oid=66008989&loc=d3e5258-111524

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Amount of investment of impaired financing receivables with related allowance for credit losses and without a related allowance for credit losses.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 14A
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=66008989&loc=SL6953550-111524

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Amount of allowance for credit losses related to recorded investment.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Recorded Investment
-URI http://asc.fasb.org/extlink&oid=65880944

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
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-Paragraph 15
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-URI http://asc.fasb.org/extlink&oid=66008989&loc=d3e5212-111524

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