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	Jun. 30, 2016	Dec. 31, 2015	Jun. 30, 2015
Financing Receivable, Impaired [Line Items]
Loans individually evaluated for impairment	$ 38,904	$ 28,601	$ 16,854
Loans collectively evaluated for impairment	3,244,751	3,288,449	2,925,370
Total	3,283,655	3,317,050	2,942,224
Commercial [Member]
Financing Receivable, Impaired [Line Items]
Loans individually evaluated for impairment	17,254	8,761	3,606
Loans collectively evaluated for impairment	644,405	687,402	646,303
Total	661,659	696,163	649,909
Agriculture [Member]
Financing Receivable, Impaired [Line Items]
Loans individually evaluated for impairment	20	97	118
Loans collectively evaluated for impairment	80,792	102,254	92,199
Total	80,812	102,351	92,317
Real Estate [Member]
Financing Receivable, Impaired [Line Items]
Loans individually evaluated for impairment	20,435	18,766	12,570
Loans collectively evaluated for impairment	2,133,953	2,117,467	1,825,918
Total	2,154,388	2,136,233	1,838,488
Consumer [Member]
Financing Receivable, Impaired [Line Items]
Loans individually evaluated for impairment	1,195	977	560
Loans collectively evaluated for impairment	385,601	381,326	360,950
Total	$ 386,796	$ 382,303	$ 361,510

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=66008989&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=65884687&loc=d3e4428-111522

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