Allowance for Loan Losses and Credit Quality of Loans, Allowance for Loan Losses (Details) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance as of Beginning of Period	$ 71,184,000	$ 70,734,000	$ 69,334,000	$ 71,334,000
Charge-offs	(7,609,000)	(5,830,000)	(20,478,000)	(17,052,000)
Recoveries	1,411,000	1,075,000	4,070,000	3,123,000
Provision	5,198,000	4,755,000	17,258,000	13,329,000
Balance as of Ending of Period	70,184,000	70,734,000	70,184,000	70,734,000
Allowance for loan losses for acquired loan portfolio			0	0
Provision expense and net charge-offs related to acquired loans	47,000		300,000
Allowance loan losses [Abstract]
Allowance for loan losses	70,184,000	70,734,000	70,184,000	70,734,000
Allowance for loans individually evaluated for impairment	1,100,000		1,100,000		2,848,000
Allowance for loans collectively evaluated for impairment	69,084,000		69,084,000		66,486,000
Ending balance of loans	5,367,231,000		5,367,231,000		4,277,616,000
Ending balance of loans individually evaluated for impairment	15,323,000		15,323,000		11,972,000
Ending balance of acquired loans	1,001,766,000		1,001,766,000
Ending balance of loans collectively evaluated for impairment	4,350,142,000		4,350,142,000		4,265,644,000
Commercial Loans [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance as of Beginning of Period	37,704,000	37,495,000	35,624,000	38,831,000
Charge-offs	(3,489,000)	(1,904,000)	(8,009,000)	(4,685,000)
Recoveries	602,000	492,000	1,485,000	1,180,000
Provision	980,000	1,734,000	6,697,000	2,491,000
Balance as of Ending of Period	35,797,000	37,817,000	35,797,000	37,817,000
Allowance loan losses [Abstract]
Allowance for loan losses	35,797,000	37,817,000	35,797,000	37,817,000
Allowance for loans individually evaluated for impairment	1,100,000		1,100,000		2,848,000
Allowance for loans collectively evaluated for impairment	34,697,000		34,697,000		32,776,000
Ending balance of loans	2,378,848,000		2,378,848,000		2,003,371,000
Ending balance of loans individually evaluated for impairment	15,323,000		15,323,000		11,972,000
Ending balance of acquired loans	432,806,000		432,806,000
Ending balance of loans collectively evaluated for impairment	1,930,719,000		1,930,719,000		1,991,399,000
Commercial Loans [Member] | Commercial Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loan portfolio segments			3
Real estate loan amount, percentage of appraised value or purchase price of the property, maximum (in hundredths)	80.00%		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%		75.00%
Allowance loan losses [Abstract]
Ending balance of loans					37,377,000
Commercial Loans [Member] | Business Banking [Member]
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	26,456,000	27,235,000	27,162,000	26,049,000
Charge-offs	(3,746,000)	(3,446,000)	(11,122,000)	(11,237,000)
Recoveries	733,000	574,000	2,363,000	1,918,000
Provision	3,744,000	2,213,000	8,784,000	9,846,000
Balance as of Ending of Period	27,187,000	26,576,000	27,187,000	26,576,000
Allowance loan losses [Abstract]
Allowance for loan losses	27,187,000	26,576,000	27,187,000	26,576,000
Allowance for loans individually evaluated for impairment	0		0		0
Allowance for loans collectively evaluated for impairment	27,187,000		27,187,000		27,162,000
Ending balance of loans	1,960,225,000		1,960,225,000		1,623,138,000
Ending balance of loans individually evaluated for impairment	0		0		0
Ending balance of acquired loans	243,469,000		243,469,000
Ending balance of loans collectively evaluated for impairment	1,716,756,000		1,716,756,000		1,623,138,000
Consumer Loans [Member] | Business Banking [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Business banking loans, maximum amount	500,000		500,000
Consumer Loans [Member] | Indirect [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percentage of automobile financing to indirect relationships with dealers (in hundredths)	75.00%		75.00%
Principal repayment term of loan, minimum			3 years
Principal repayment term of loan, maximum			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%		85.00%
Consumer Loans [Member] | Direct [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal repayment term of loan, minimum			1 year
Principal repayment term of loan, maximum			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%		85.00%
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance as of Beginning of Period	6,806,000	5,943,000	6,252,000	6,249,000
Charge-offs	(374,000)	(480,000)	(1,347,000)	(1,130,000)
Recoveries	76,000	9,000	179,000	25,000
Provision	402,000	809,000	1,826,000	1,137,000
Balance as of Ending of Period	6,910,000	6,281,000	6,910,000	6,281,000
Allowance loan losses [Abstract]
Allowance for loan losses	6,910,000	6,281,000	6,910,000	6,281,000
Allowance for loans individually evaluated for impairment	0		0		0
Allowance for loans collectively evaluated for impairment	6,910,000		6,910,000		6,252,000
Ending balance of loans	1,028,158,000		1,028,158,000		651,107,000
Ending balance of loans individually evaluated for impairment	0		0		0
Ending balance of acquired loans	325,491,000		325,491,000
Ending balance of loans collectively evaluated for impairment	702,667,000		702,667,000		651,107,000
Unallocated [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance as of Beginning of Period	218,000	61,000	296,000	205,000
Charge-offs	0	0	0	0
Recoveries	0	0	43,000	0
Provision	72,000	(1,000)	(49,000)	(145,000)
Balance as of Ending of Period	290,000	60,000	290,000	60,000
Allowance loan losses [Abstract]
Allowance for loan losses	290,000	60,000	290,000	60,000
Allowance for loans collectively evaluated for impairment	$ 290,000		$ 290,000		$ 296,000