Allowance for Loan Losses and Credit Quality of Loans, Allowance for Loan Losses (Details) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Dec. 31, 2012
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance as of Beginning of Period	$ 68,734,000	$ 71,334,000	$ 69,334,000	$ 71,334,000
Charge-offs	(5,153,000)	(5,680,000)	(12,869,000)	(11,220,000)
Recoveries	1,201,000	977,000	2,659,000	2,046,000
Provision	6,402,000	4,103,000	12,060,000	8,574,000
Balance as of Ending of Period	71,184,000	70,734,000	71,184,000	70,734,000
Allowance for loan losses for acquired loan portfolio			0	0
Provision expense and net charge-offs related to acquired loans	200,000		400,000
Allowance loan losses [Abstract]
Allowance for loan losses	71,184,000	70,734,000	71,184,000	70,734,000
Allowance for loans and leases individually evaluated for impairment	2,032,000		2,032,000		2,848,000
Allowance for loans and leases collectively evaluated for impairment	69,152,000		69,152,000		66,486,000
Ending balance of loans and leases	5,290,710,000		5,290,710,000		4,277,616,000
Ending balance of loans and leases individually evaluated for impairment	12,887,000		12,887,000		11,972,000
Ending balance of acquired loans and leases	1,068,896,000		1,068,896,000
Ending balance of loans and leases collectively evaluated for impairment	4,208,927,000		4,208,927,000		4,265,644,000
Commercial Loans [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance as of Beginning of Period	35,358,000	37,787,000	35,624,000	38,831,000
Charge-offs	(1,198,000)	(1,653,000)	(4,520,000)	(2,783,000)
Recoveries	416,000	303,000	883,000	688,000
Provision	3,128,000	1,058,000	5,717,000	759,000
Balance as of Ending of Period	37,704,000	37,495,000	37,704,000	37,495,000
Allowance loan losses [Abstract]
Allowance for loan losses	37,704,000	37,495,000	37,704,000	37,495,000
Allowance for loans and leases individually evaluated for impairment	2,032,000		2,032,000		2,848,000
Allowance for loans and leases collectively evaluated for impairment	35,672,000		35,672,000		32,776,000
Ending balance of loans and leases	2,368,897,000		2,368,897,000		2,003,371,000
Ending balance of loans and leases individually evaluated for impairment	12,887,000		12,887,000		11,972,000
Ending balance of acquired loans and leases	460,995,000		460,995,000
Ending balance of loans and leases collectively evaluated for impairment	1,895,015,000		1,895,015,000		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 and leases					37,377,000
Commercial Loans [Member] | Business Banking [Member]
Allowance loan losses [Abstract]
Ending balance of loans and leases					365,960,000
Consumer Loans [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance as of Beginning of Period	26,285,000	26,790,000	27,162,000	26,049,000
Charge-offs	(3,653,000)	(3,735,000)	(7,376,000)	(7,787,000)
Recoveries	696,000	667,000	1,673,000	1,342,000
Provision	3,128,000	3,513,000	4,997,000	7,631,000
Balance as of Ending of Period	26,456,000	27,235,000	26,456,000	27,235,000
Allowance loan losses [Abstract]
Allowance for loan losses	26,456,000	27,235,000	26,456,000	27,235,000
Allowance for loans and leases individually evaluated for impairment	0		0		0
Allowance for loans and leases collectively evaluated for impairment	26,456,000		26,456,000		27,162,000
Ending balance of loans and leases	1,920,171,000		1,920,171,000		1,623,138,000
Ending balance of loans and leases individually evaluated for impairment	0		0		0
Ending balance of acquired loans and leases	269,151,000		269,151,000
Ending balance of loans and leases collectively evaluated for impairment	1,651,020,000		1,651,020,000		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 (in years)			3 years
Principal repayment term of loan, maximum (in years)			6 years
Allowance loan losses [Abstract]
Ending balance of loans and leases					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 and leases					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	6,708,000	6,520,000	6,252,000	6,249,000
Charge-offs	(302,000)	(292,000)	(973,000)	(650,000)
Recoveries	89,000	7,000	103,000	16,000
Provision	311,000	(292,000)	1,424,000	328,000
Balance as of Ending of Period	6,806,000	5,943,000	6,806,000	5,943,000
Allowance loan losses [Abstract]
Allowance for loan losses	6,806,000	5,943,000	6,806,000	5,943,000
Allowance for loans and leases individually evaluated for impairment	0		0		0
Allowance for loans and leases collectively evaluated for impairment	6,806,000		6,806,000		6,252,000
Ending balance of loans and leases	1,001,642,000		1,001,642,000		651,107,000
Ending balance of loans and leases individually evaluated for impairment	0		0		0
Ending balance of acquired loans and leases	338,750,000		338,750,000
Ending balance of loans and leases collectively evaluated for impairment	662,892,000		662,892,000		651,107,000
Unallocated [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance as of Beginning of Period	383,000	237,000	296,000	205,000
Charge-offs	0	0	0	0
Recoveries	0	0	0	0
Provision	(165,000)	(176,000)	(78,000)	(144,000)
Balance as of Ending of Period	218,000	61,000	218,000	61,000
Allowance loan losses [Abstract]
Allowance for loan losses	218,000	61,000	218,000	61,000
Allowance for loans and leases collectively evaluated for impairment	$ 218,000		$ 218,000		$ 296,000