Loan Portfolio and Credit Quality Loans by Portfolio Segment (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	$ 6,602,327	$ 6,505,028
Commercial and industrial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	531,093	520,992
Commercial Tax Exempt [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	420,757	418,698
Commercial and Industrial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	951,850	939,690
Commercial real estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	2,465,003	2,440,220
Construction and land [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	165,240	164,990
Residential [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	2,737,369	2,682,533
Home equity [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	94,331	99,958
Consumer and other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	$ 188,534	$ 177,637