Allowance for Loan Losses and Credit Quality of Loans, Past Due Loans (Details) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
Financing Receivable, Recorded Investment, Aging [Abstract]
Minimum number of days past due for nonaccrual loan status (in days)	90 days
Financing Receivable, Recorded Investment, Past Due [Abstract]
31 - 60 Days Past Due Accruing	$ 22,852,000	$ 25,351,000
61 - 90 Days Past Due Accruing	5,277,000	5,278,000
Greater Than 90 Days Past Due Accruing	2,448,000	3,190,000
Total Past Due Accruing	30,577,000	33,819,000
Non-Accrual	39,676,000	38,290,000
Current	4,207,363,000	3,728,094,000
Recorded Total Loans and Leases	4,277,616,000	3,800,203,000
Within non accrual loans [Member]
Financing Receivable, Recorded Investment, Past Due [Abstract]
Troubled debt restructured loans amount	2,200,000	4,000,000
Commercial Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Abstract]
Recorded Total Loans and Leases	2,003,371,000	1,702,577,000
Commercial Loans [Member] | Commercial [Member]
Financing Receivable, Recorded Investment, Past Due [Abstract]
31 - 60 Days Past Due Accruing	0	663,000
61 - 90 Days Past Due Accruing	0	50,000
Greater Than 90 Days Past Due Accruing	0	0
Total Past Due Accruing	0	713,000
Non-Accrual	4,985,000	1,699,000
Current	556,496,000	508,662,000
Recorded Total Loans and Leases	561,481,000	511,074,000
Commercial Loans [Member] | Commercial Real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Abstract]
31 - 60 Days Past Due Accruing	126,000	1,942,000
61 - 90 Days Past Due Accruing	0	0
Greater Than 90 Days Past Due Accruing	0	0
Total Past Due Accruing	126,000	1,942,000
Non-Accrual	7,977,000	4,868,000
Current	966,692,000	828,089,000
Recorded Total Loans and Leases	974,795,000	834,899,000
Commercial Loans [Member] | Agricultural [Member]
Financing Receivable, Recorded Investment, Past Due [Abstract]
31 - 60 Days Past Due Accruing	22,000	77,000
61 - 90 Days Past Due Accruing	0	13,000
Greater Than 90 Days Past Due Accruing	0	0
Total Past Due Accruing	22,000	90,000
Non-Accrual	699,000	3,307,000
Current	63,037,000	63,140,000
Recorded Total Loans and Leases	63,758,000	66,537,000
Commercial Loans [Member] | Agricultural Real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Abstract]
31 - 60 Days Past Due Accruing	108,000	0
61 - 90 Days Past Due Accruing	0	0
Greater Than 90 Days Past Due Accruing	103,000	50,000
Total Past Due Accruing	211,000	50,000
Non-Accrual	1,038,000	2,067,000
Current	36,128,000	31,809,000
Recorded Total Loans and Leases	37,377,000	33,926,000
Commercial Loans [Member] | Business Banking [Member]
Financing Receivable, Recorded Investment, Past Due [Abstract]
31 - 60 Days Past Due Accruing	3,019,000	1,871,000
61 - 90 Days Past Due Accruing	708,000	1,024,000
Greater Than 90 Days Past Due Accruing	45,000	0
Total Past Due Accruing	3,772,000	2,895,000
Non-Accrual	6,738,000	7,446,000
Current	355,450,000	245,800,000
Recorded Total Loans and Leases	365,960,000	256,141,000
Commercial Loans [Member] | Total Commercial Loan [Member]
Financing Receivable, Recorded Investment, Past Due [Abstract]
31 - 60 Days Past Due Accruing	3,275,000	4,553,000
61 - 90 Days Past Due Accruing	708,000	1,087,000
Greater Than 90 Days Past Due Accruing	148,000	50,000
Total Past Due Accruing	4,131,000	5,690,000
Non-Accrual	21,437,000	19,387,000
Current	1,977,803,000	1,677,500,000
Recorded Total Loans and Leases	2,003,371,000	1,702,577,000
Consumer Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Abstract]
Recorded Total Loans and Leases	1,623,138,000	1,516,115,000
Consumer Loans [Member] | Indirect [Member]
Financing Receivable, Recorded Investment, Past Due [Abstract]
31 - 60 Days Past Due Accruing	10,956,000	12,141,000
61 - 90 Days Past Due Accruing	2,477,000	2,584,000
Greater Than 90 Days Past Due Accruing	1,205,000	1,283,000
Total Past Due Accruing	14,638,000	16,008,000
Non-Accrual	1,557,000	1,550,000
Current	964,802,000	855,545,000
Recorded Total Loans and Leases	980,997,000	873,103,000
Consumer Loans [Member] | Home Equity Member]
Financing Receivable, Recorded Investment, Past Due [Abstract]
31 - 60 Days Past Due Accruing	6,065,000	5,823,000
61 - 90 Days Past Due Accruing	1,223,000	1,277,000
Greater Than 90 Days Past Due Accruing	681,000	954,000
Total Past Due Accruing	7,969,000	8,054,000
Non-Accrual	7,247,000	7,931,000
Current	560,066,000	553,660,000
Recorded Total Loans and Leases	575,282,000	569,645,000
Consumer Loans [Member] | Direct [Member]
Financing Receivable, Recorded Investment, Past Due [Abstract]
31 - 60 Days Past Due Accruing	717,000	831,000
61 - 90 Days Past Due Accruing	144,000	191,000
Greater Than 90 Days Past Due Accruing	84,000	140,000
Total Past Due Accruing	945,000	1,162,000
Non-Accrual	266,000	378,000
Current	65,648,000	71,827,000
Recorded Total Loans and Leases	66,859,000	73,367,000
Consumer Loans [Member] | Total Consumer Loan [Member]
Financing Receivable, Recorded Investment, Past Due [Abstract]
31 - 60 Days Past Due Accruing	17,738,000	18,795,000
61 - 90 Days Past Due Accruing	3,844,000	4,052,000
Greater Than 90 Days Past Due Accruing	1,970,000	2,377,000
Total Past Due Accruing	23,552,000	25,224,000
Non-Accrual	9,070,000	9,859,000
Current	1,590,516,000	1,481,032,000
Recorded Total Loans and Leases	1,623,138,000	1,516,115,000
Residential Real Estate Mortgages [Member]
Financing Receivable, Recorded Investment, Past Due [Abstract]
31 - 60 Days Past Due Accruing	1,839,000	2,003,000
61 - 90 Days Past Due Accruing	725,000	139,000
Greater Than 90 Days Past Due Accruing	330,000	763,000
Total Past Due Accruing	2,894,000	2,905,000
Non-Accrual	9,169,000	9,044,000
Current	639,044,000	569,562,000
Recorded Total Loans and Leases	$ 651,107,000	$ 581,511,000