Allowance for Loan and Lease Losses (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
Mar. 31, 2015	Mar. 31, 2014
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Balance	$ 41,449	$ 41,685
Charge-offs	(2,004)	(10,658)
Recoveries	738	1,215
Provision	1,671us-gaap_ProvisionForLoanAndLeaseLosses	6,331us-gaap_ProvisionForLoanAndLeaseLosses
Ending Balance	41,854	38,573
Commercial
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Balance	11,909	13,099
Charge-offs	(274)	(6,917)
Recoveries	320	199
Provision	(267)	5,252
Ending Balance	11,688	11,633
Commercial real estate
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Balance	15,898	14,152
Charge-offs	(333)	(923)
Recoveries	126	780
Provision	944	(999)
Ending Balance	16,635	13,010
Agricultural
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Balance	3,295	2,992
Charge-offs	(276)	(1,511)
Recoveries	22	2
Provision	175	1,088
Ending Balance	3,216	2,571
Residential real estate
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Balance	3,741	3,720
Charge-offs	(58)	(149)
Recoveries	37	26
Provision	25	74
Ending Balance	3,745	3,671
Consumer
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Balance	6,606	7,722
Charge-offs	(1,063)	(1,158)
Recoveries	233	208
Provision	794	604
Ending Balance	6,570	7,376
Unallocated
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Balance	0	0
Charge-offs	0	0
Recoveries	0	0
Provision	0	312
Ending Balance	$ 0	$ 312

A valuation allowance for financing receivables that are expected to be uncollectible.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 50

-Paragraph 11B

-Subparagraph (c)(1)

-URI http://asc.fasb.org/extlink&oid=35738203&loc=SL6953423-111524

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Amount of recovery of financing receivables doubtful of collection that were previously charged off.

+ 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 (c)(4)

-URI http://asc.fasb.org/extlink&oid=35738203&loc=SL6953423-111524

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Amount of direct write-downs of financing receivables charged against the allowance.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 50

-Paragraph 11B

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-URI http://asc.fasb.org/extlink&oid=35738203&loc=SL6953423-111524

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- Definition

Amount of expense related to estimated loss from loan and lease transactions.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 28

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3602-108585

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 04

-Paragraph 11

-Article 9

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 225

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