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, 2016	Dec. 31, 2015	Mar. 31, 2015
Financing Receivable, Impaired [Line Items]
Loans individually evaluated for impairment	$ 27,175	$ 28,601	$ 18,935
Loans collectively evaluated for impairment	3,255,024	3,288,449	2,909,541
Total	3,282,199	3,317,050	2,928,476
Commercial [Member]
Financing Receivable, Impaired [Line Items]
Loans individually evaluated for impairment	7,392	8,761	3,098
Loans collectively evaluated for impairment	662,133	687,402	648,625
Total	669,525	696,163	651,723
Agriculture [Member]
Financing Receivable, Impaired [Line Items]
Loans individually evaluated for impairment	19	97	197
Loans collectively evaluated for impairment	87,471	102,254	90,413
Total	87,490	102,351	90,610
Real Estate Member]
Financing Receivable, Impaired [Line Items]
Loans individually evaluated for impairment	18,473	18,766	14,983
Loans collectively evaluated for impairment	2,131,659	2,117,467	1,811,596
Total	2,150,132	2,136,233	1,826,579
Consumer [Member]
Financing Receivable, Impaired [Line Items]
Loans individually evaluated for impairment	1,291	977	657
Loans collectively evaluated for impairment	373,761	381,326	358,907
Total	$ 375,052	$ 382,303	$ 359,564

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

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=49124121&loc=SL6953423-111524

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

+ References

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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=28367877&loc=d3e4428-111522

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