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 $)	Mar. 31, 2015	Dec. 31, 2014	Mar. 31, 2014
Financing Receivable, Impaired [Line Items]
Loans individually evaluated for impairment	$ 18,935,000	$ 20,194,000
Loans collectively evaluated for impairment	2,909,541,000	2,908,994,000
Total	2,928,476,000	2,929,188,000	2,688,288,000
Commercial [Member]
Financing Receivable, Impaired [Line Items]
Loans individually evaluated for impairment	3,098,000	3,193,000
Loans collectively evaluated for impairment	648,625,000	636,761,000
Total	651,723,000	639,954,000
Agriculture [Member]
Financing Receivable, Impaired [Line Items]
Loans individually evaluated for impairment	197,000	165,000
Loans collectively evaluated for impairment	90,413,000	105,529,000
Total	90,610,000	105,694,000
Real Estate [Member]
Financing Receivable, Impaired [Line Items]
Loans individually evaluated for impairment	14,983,000	16,218,000
Loans collectively evaluated for impairment	1,811,596,000	1,806,636,000
Total	1,826,579,000	1,822,854,000
Consumer [Member]
Financing Receivable, Impaired [Line Items]
Loans individually evaluated for impairment	657,000	618,000
Loans collectively evaluated for impairment	358,907,000	360,068,000
Total	$ 359,564,000	$ 360,686,000

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=35738203&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=28367877&loc=d3e4428-111522

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