Allowance for Loan Losses and Credit Quality of Loans, Troubled Debt Restructurings (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
Financing Receivable, Modifications [Line Items]
Period of sustained repayment performance for nonperforming TDRs to be returned to performing status (in months)	6 months
Subsequent Default [Member] | Home Equity [Member]
Financing Receivable, Modifications [Line Items]
Number of contracts	1
Pre-modification oustanding recorded investment	$ 0.1
Business Banking [Member]
Financing Receivable, Modifications [Line Items]
Number of contracts	1
Pre-modification oustanding recorded investment	0.1
Home Equity [Member]
Financing Receivable, Modifications [Line Items]
Number of contracts	18	25
Pre-modification oustanding recorded investment	0.9	2.4
Home Equity [Member] | Subsequent Default [Member]
Financing Receivable, Modifications [Line Items]
Number of contracts		7
Pre-modification oustanding recorded investment		0.6
Commercial Loans [Member]
Financing Receivable, Modifications [Line Items]
Number of contracts	4	3
Pre-modification oustanding recorded investment	6.6	0.7
Commercial Loans [Member] | Subsequent Default [Member]
Financing Receivable, Modifications [Line Items]
Number of contracts		2
Pre-modification oustanding recorded investment		0.7
Residential Real Estate Mortgages [Member]
Financing Receivable, Modifications [Line Items]
Number of contracts	12	4
Pre-modification oustanding recorded investment	1.2	0.8
Residential Real Estate Mortgages [Member] | Subsequent Default [Member]
Financing Receivable, Modifications [Line Items]
Number of contracts	2	2
Pre-modification oustanding recorded investment	$ 0.3	$ 0.4