Allowance for Loan Losses and Credit Quality of Loans, Allowance for Loan Losses (Details) (USD $)	3 Months Ended
	Mar. 31, 2013 Segment	Mar. 31, 2012	Dec. 31, 2012
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	$ 69,334,000	$ 71,334,000
Charge-offs	(7,716,000)	(5,540,000)
Recoveries	1,458,000	1,069,000
Provision	5,658,000	4,471,000
Balance as of Ending of Period	68,734,000	71,334,000
Allowance for loan losses for the acquired loan portfolio	0	0
Provision expense and net charge-offs related to acquired loans	200,000
Allowance loan losses [Abstract]
Allowance for loan losses	68,734,000	71,334,000
Allowance for loans individually evaluated for impairment	607,000		2,848,000
Allowance for loans collectively evaluated for impairment	68,127,000		66,486,000
Ending balance of loans	5,195,033,000		4,277,616,000
Ending balance of loans individually evaluated for impairment	9,690,000		11,972,000
Ending balance of acquired loans	1,123,133,000
Ending balance of loans collectively evaluated for impairment	4,062,210,000		4,265,644,000
Commercial Loans [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance as of Beginning of Period	35,624,000	38,831,000
Charge-offs	(3,322,000)	(1,130,000)
Recoveries	467,000	385,000
Provision	2,589,000	299,000
Balance as of Ending of Period	35,358,000	37,787,000
Allowance loan losses [Abstract]
Allowance for loan losses	35,358,000	37,787,000
Allowance for loans individually evaluated for impairment	607,000		2,848,000
Allowance for loans collectively evaluated for impairment	34,751,000		32,776,000
Ending balance of loans	2,306,954,000		2,003,371,000
Ending balance of loans individually evaluated for impairment	9,690,000		11,972,000
Ending balance of acquired loans	466,673,000
Ending balance of loans collectively evaluated for impairment	1,830,591,000		1,991,399,000
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			37,377,000
Commercial Loans [Member] | Business Banking [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Business banking loans, maximum amount	500,000
Allowance loan losses [Abstract]
Ending balance of loans			365,960,000
Consumer Loans [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance as of Beginning of Period	27,162,000	26,049,000
Charge-offs	(3,723,000)	(4,052,000)
Recoveries	977,000	675,000
Provision	1,869,000	4,118,000
Balance as of Ending of Period	26,285,000	26,790,000
Allowance loan losses [Abstract]
Allowance for loan losses	26,285,000	26,790,000
Allowance for loans individually evaluated for impairment	0		0
Allowance for loans collectively evaluated for impairment	26,285,000		27,162,000
Ending balance of loans	1,891,154,000		1,623,138,000
Ending balance of loans individually evaluated for impairment	0		0
Ending balance of acquired loans	294,780,000
Ending balance of loans collectively evaluated for impairment	1,596,374,000		1,623,138,000
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			980,997,000
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			66,859,000
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	6,252,000	6,249,000
Charge-offs	(671,000)	(358,000)
Recoveries	14,000	9,000
Provision	1,113,000	620,000
Balance as of Ending of Period	6,708,000	6,520,000
Allowance loan losses [Abstract]
Allowance for loan losses	6,708,000	6,520,000
Allowance for loans individually evaluated for impairment	0		0
Allowance for loans collectively evaluated for impairment	6,708,000		6,252,000
Ending balance of loans	996,925,000		651,107,000
Ending balance of loans individually evaluated for impairment	0		0
Ending balance of acquired loans	361,680,000
Ending balance of loans collectively evaluated for impairment	635,245,000		651,107,000
Unallocated [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance as of Beginning of Period	296,000	205,000
Charge-offs	0	0
Recoveries	0	0
Provision	87,000	32,000
Balance as of Ending of Period	383,000	237,000
Allowance loan losses [Abstract]
Allowance for loan losses	383,000	237,000
Allowance for loans collectively evaluated for impairment	$ 383,000		$ 296,000