Allowance for Loan Losses and Credit Quality of Loans, Allowance for Loan Losses (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loan portfolio segments	3
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance as of Beginning of Period	$ 71,334	$ 71,234	$ 66,550
Charge-offs	(26,504)	(24,488)	(29,837)
Recoveries	4,235	3,851	4,712
Provision	20,269	20,737	29,809
Balance as of Ending of Period	69,334	71,334	71,234
Allowance loan losses [Abstract]
Allowance for loan losses	69,334	71,334	71,234
Allowance for loans individually evaluated for impairment	2,848	175
Allowance for loans collectively evaluated for impairment	66,486	71,159
Ending balance of loans	4,277,616	3,800,203
Ending balance of loans individually evaluated for impairment	11,972	6,219
Ending balance of loans collectively evaluated for impairment	4,265,644	3,793,984
Commercial Loans [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance as of Beginning of Period	38,831	40,101	36,599
Charge-offs	(8,750)	(8,969)	(12,969)
Recoveries	1,641	1,438	1,922
Provision	3,902	6,261	14,549
Balance as of Ending of Period	35,624	38,831	40,101
Allowance loan losses [Abstract]
Allowance for loan losses	35,624	38,831	40,101
Allowance for loans individually evaluated for impairment	2,848	175
Allowance for loans collectively evaluated for impairment	32,776	38,656
Ending balance of loans	2,003,371	1,702,577
Ending balance of loans individually evaluated for impairment	11,972	6,219
Ending balance of loans collectively evaluated for impairment	1,991,399	1,696,358
Commercial Loans [Member] \| Commercial Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Real estate loan amount, percentage of appraised value or purchase price of the property, maximum (in hundredths)	80.00%
Commercial Loans [Member] \| Agricultural Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Real estate loan amount, percentage of appraised value or purchase price of the property, maximum (in hundredths)	75.00%
Allowance loan losses [Abstract]
Ending balance of loans	37,377	33,926
Commercial Loans [Member] \| Business Banking [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Business banking loans, maximum amount	500
Allowance loan losses [Abstract]
Ending balance of loans	365,960	256,141
Consumer Loans [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance as of Beginning of Period	26,049	26,126	26,664
Charge-offs	(15,848)	(14,209)	(15,692)
Recoveries	2,556	2,406	2,747
Provision	14,405	11,726	12,407
Balance as of Ending of Period	27,162	26,049	26,126
Allowance loan losses [Abstract]
Allowance for loan losses	27,162	26,049	26,126
Allowance for loans individually evaluated for impairment	0	0
Allowance for loans collectively evaluated for impairment	27,162	26,049
Ending balance of loans	1,623,138	1,516,115
Ending balance of loans individually evaluated for impairment	0	0
Ending balance of loans collectively evaluated for impairment	1,623,138	1,516,115
Consumer Loans [Member] \| Indirect [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percentage of automobile financing to indirect relationships with dealers (in hundredths)	70.00%
Principal repayment term of loan, minimum (in years)	3 years
Principal repayment term of loan, maximum (in years)	6 years
Allowance loan losses [Abstract]
Ending balance of loans	980,997	873,103
Consumer Loans [Member] \| Home Equity [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Home equity loan amount, percentage of equity in home, maximum (in hundredths)	85.00%
Consumer Loans [Member] \| Direct [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal repayment term of loan, minimum (in years)	1 year
Principal repayment term of loan, maximum (in years)	10 years
Allowance loan losses [Abstract]
Ending balance of loans	66,859	73,367
Residential Real Estate Mortgages [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Real estate loan amount, percentage of appraised value or purchase price of the property, maximum (in hundredths)	85.00%
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance as of Beginning of Period	6,249	4,627	3,002
Charge-offs	(1,906)	(1,310)	(1,176)
Recoveries	38	7	43
Provision	1,871	2,925	2,758
Balance as of Ending of Period	6,252	6,249	4,627
Allowance loan losses [Abstract]
Allowance for loan losses	6,252	6,249	4,627
Allowance for loans individually evaluated for impairment	0	0
Allowance for loans collectively evaluated for impairment	6,252	6,249
Ending balance of loans	651,107	581,511
Ending balance of loans individually evaluated for impairment	0	0
Ending balance of loans collectively evaluated for impairment	651,107	581,511
Unallocated [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance as of Beginning of Period	205	380	285
Charge-offs	0	0	0
Recoveries	0	0	0
Provision	91	(175)	95
Balance as of Ending of Period	296	205	380
Allowance loan losses [Abstract]
Allowance for loan losses	296	205	380
Allowance for loans collectively evaluated for impairment	$ 296	$ 205

Represents the maximum repayment term of the debt instrument.

No definition available.

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Represents the minimum repayment term of the debt instrument.

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- Definition

This element represents the maximum percentage of the equity in home , for the originated amounts of home equity loans.

+ References

No definition available.

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- Definition

This element represents the maximum percentage of the appraised value or purchase price of a property, for the originated amounts of real estate loans.

+ References

No definition available.

+ Details

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- Definition

This element represents the number of portfolio segments within the loan portfolio.

+ References

No definition available.

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- Definition

Represents the percentage of automobile financing to indirect relationships the Company maintains with dealers.

+ References

No definition available.

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- Definition

This element represents the maximum loan amount available for small business customers.

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No definition available.

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- Definition

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

+ 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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- Definition

The valuation allowance for financing receivables that are expected to be uncollectible that were collectively evaluated for impairment.

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- Definition

The valuation allowance for financing receivables that are expected to be uncollectible that were individually evaluated for impairment.

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- Definition

Reduction to the allowance for credit losses related to collections on financing receivables which have been partially or fully charged off as bad debts.

+ References

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- Definition

Reduction to the allowance for credit losses related to financing receivables deemed uncollectible.

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- Definition

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

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

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- Definition

Reflects the aggregate gross carrying amount of all categories of loans and leases held in portfolio, after deducting deferred income, but before deducting the allowance for losses on loans and leases.

+ References

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-Publisher FASB

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-Paragraph 4

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-Publisher AICPA

-Name Statement of Position (SOP)

-Number 01-6

-Paragraph 13

-Subparagraph e

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-LegacyDoc This is a non-GAAP reference that was included in the 2009 taxonomy. It will be removed from future versions of this taxonomy.

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- Definition

The sum of the periodic provision charged to earnings, based on an assessment of uncollectibility from the counterparty on account of loan, lease or other credit losses, to reduce these accounts to the amount that approximates their net realizable value.

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