Loan Portfolio and Credit Quality Loan Participation Amounts (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Loan Participation Amounts by Loan Type [Line Items]
Loan Participation Serviced for Others Amount	$ 106,025	$ 115,912
Loan Serviced for Others Amount	41,207	41,440
Commercial and industrial [Member]
Loan Participation Amounts by Loan Type [Line Items]
Loan Participation Serviced for Others Amount	8,397	8,484
Commercial Tax Exempt [Member]
Loan Participation Amounts by Loan Type [Line Items]
Loan Participation Serviced for Others Amount	19,582	19,805
Commercial real estate [Member]
Loan Participation Amounts by Loan Type [Line Items]
Loan Participation Serviced for Others Amount	43,965	49,783
Construction and land [Member]
Loan Participation Amounts by Loan Type [Line Items]
Loan Participation Serviced for Others Amount	34,081	37,840
Residential [Member]
Loan Participation Amounts by Loan Type [Line Items]
Loan Serviced for Others Amount	$ 41,207	$ 41,440