Allowance for Loan Losses and Credit Quality of Loans, Past Due Loans (Details) (USD $)	Sep. 30, 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	$ 21,105,000	$ 25,351,000
61 - 90 Days Past Due Accruing	3,759,000	5,278,000
Greater Than 90 Days Past Due Accruing	2,963,000	3,190,000
Total Past Due Accruing	27,827,000	33,819,000
Non-Accrual	42,661,000	38,290,000
Current	4,180,631,000	3,728,094,000
Recorded Total Loans	4,251,119,000	3,800,203,000
Within non accrual loans [Member]
Financing Receivable, Recorded Investment, Past Due [Abstract]
Troubled debt restructured loans amount	2,300,000	4,000,000
Commercial Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Abstract]
31 - 60 Days Past Due Accruing	3,276,000	4,553,000
61 - 90 Days Past Due Accruing	842,000	1,087,000
Greater Than 90 Days Past Due Accruing	80,000	50,000
Total Past Due Accruing	4,198,000	5,690,000
Non-Accrual	23,050,000	19,387,000
Current	1,965,643,000	1,677,500,000
Recorded Total Loans	1,992,891,000	1,702,577,000
Commercial Loans [Member] | Commercial [Member]
Financing Receivable, Recorded Investment, Past Due [Abstract]
31 - 60 Days Past Due Accruing	707,000	663,000
61 - 90 Days Past Due Accruing	149,000	50,000
Greater Than 90 Days Past Due Accruing	0	0
Total Past Due Accruing	856,000	713,000
Non-Accrual	6,915,000	1,699,000
Current	594,775,000	508,662,000
Recorded Total Loans	602,546,000	511,074,000
Commercial Loans [Member] | Commercial Real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Abstract]
31 - 60 Days Past Due Accruing	1,455,000	1,942,000
61 - 90 Days Past Due Accruing	138,000	0
Greater Than 90 Days Past Due Accruing	0	0
Total Past Due Accruing	1,593,000	1,942,000
Non-Accrual	6,432,000	4,868,000
Current	1,012,695,000	828,089,000
Recorded Total Loans	1,020,720,000	834,899,000
Commercial Loans [Member] | Agricultural [Member]
Financing Receivable, Recorded Investment, Past Due [Abstract]
31 - 60 Days Past Due Accruing	0	77,000
61 - 90 Days Past Due Accruing	0	13,000
Greater Than 90 Days Past Due Accruing	0	0
Total Past Due Accruing	0	90,000
Non-Accrual	2,587,000	3,307,000
Current	64,187,000	63,140,000
Recorded Total Loans	66,774,000	66,537,000
Commercial Loans [Member] | Agricultural Real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Abstract]
31 - 60 Days Past Due Accruing	0	0
61 - 90 Days Past Due Accruing	0	0
Greater Than 90 Days Past Due Accruing	0	50,000
Total Past Due Accruing	0	50,000
Non-Accrual	1,195,000	2,067,000
Current	35,348,000	31,809,000
Recorded Total Loans	36,543,000	33,926,000
Commercial Loans [Member] | Business Banking [Member]
Financing Receivable, Recorded Investment, Past Due [Abstract]
31 - 60 Days Past Due Accruing	1,114,000	1,871,000
61 - 90 Days Past Due Accruing	555,000	1,024,000
Greater Than 90 Days Past Due Accruing	80,000	0
Total Past Due Accruing	1,749,000	2,895,000
Non-Accrual	5,921,000	7,446,000
Current	258,638,000	245,800,000
Recorded Total Loans	266,308,000	256,141,000
Consumer Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Abstract]
31 - 60 Days Past Due Accruing	15,589,000	18,795,000
61 - 90 Days Past Due Accruing	2,747,000	4,052,000
Greater Than 90 Days Past Due Accruing	1,598,000	2,377,000
Total Past Due Accruing	19,934,000	25,224,000
Non-Accrual	10,086,000	9,859,000
Current	1,577,760,000	1,481,032,000
Recorded Total Loans	1,607,780,000	1,516,115,000
Consumer Loans [Member] | Indirect [Member]
Financing Receivable, Recorded Investment, Past Due [Abstract]
31 - 60 Days Past Due Accruing	9,406,000	12,141,000
61 - 90 Days Past Due Accruing	1,729,000	2,584,000
Greater Than 90 Days Past Due Accruing	978,000	1,283,000
Total Past Due Accruing	12,113,000	16,008,000
Non-Accrual	1,703,000	1,550,000
Current	948,803,000	855,545,000
Recorded Total Loans	962,619,000	873,103,000
Consumer Loans [Member] | Home Equity [Member]
Financing Receivable, Recorded Investment, Past Due [Abstract]
31 - 60 Days Past Due Accruing	5,478,000	5,823,000
61 - 90 Days Past Due Accruing	888,000	1,277,000
Greater Than 90 Days Past Due Accruing	573,000	954,000
Total Past Due Accruing	6,939,000	8,054,000
Non-Accrual	8,126,000	7,931,000
Current	561,143,000	553,660,000
Recorded Total Loans	576,208,000	569,645,000
Consumer Loans [Member] | Direct [Member]
Financing Receivable, Recorded Investment, Past Due [Abstract]
31 - 60 Days Past Due Accruing	705,000	831,000
61 - 90 Days Past Due Accruing	130,000	191,000
Greater Than 90 Days Past Due Accruing	47,000	140,000
Total Past Due Accruing	882,000	1,162,000
Non-Accrual	257,000	378,000
Current	67,814,000	71,827,000
Recorded Total Loans	68,953,000	73,367,000
Residential Real Estate Mortgages [Member]
Financing Receivable, Recorded Investment, Past Due [Abstract]
31 - 60 Days Past Due Accruing	2,240,000	2,003,000
61 - 90 Days Past Due Accruing	170,000	139,000
Greater Than 90 Days Past Due Accruing	1,285,000	763,000
Total Past Due Accruing	3,695,000	2,905,000
Non-Accrual	9,525,000	9,044,000
Current	637,228,000	569,562,000
Recorded Total Loans	$ 650,448,000	$ 581,511,000