Loan Portfolio and Credit Quality Loans by Past Due Status (Details) $ in Thousands	Mar. 31, 2020 USD ($) loan	Dec. 31, 2019 USD ($) loan
Financing Receivable, Past Due [Line Items]
Accruing Past Due	$ 14,852	$ 25,945
Nonaccrual Loans	24,314	16,103
Current Accruing Loans	7,004,172	6,934,656
Total Loans Receivable	7,043,338	6,976,704
30-59 Days Past Due
Financing Receivable, Past Due [Line Items]
Accruing Past Due	14,441	22,758
60-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Accruing Past Due	411	3,187
Current
Financing Receivable, Past Due [Line Items]
Nonaccrual Loans	14,978	9,814
30-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Nonaccrual Loans	2,056	1,148
90 Days or Greater Past Due
Financing Receivable, Past Due [Line Items]
Nonaccrual Loans	7,280	5,141
Commercial and industrial
Financing Receivable, Past Due [Line Items]
Accruing Past Due	1,493	828
Nonaccrual Loans	1,371	582
Current Accruing Loans	667,880	692,624
Total Loans Receivable	670,744	694,034
Commercial and industrial | 30-59 Days Past Due
Financing Receivable, Past Due [Line Items]
Accruing Past Due	1,401	828
Commercial and industrial | 60-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Accruing Past Due	92	0
Commercial and industrial | Current
Financing Receivable, Past Due [Line Items]
Nonaccrual Loans	381	0
Commercial and industrial | 30-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Nonaccrual Loans	0	241
Commercial and industrial | 90 Days or Greater Past Due
Financing Receivable, Past Due [Line Items]
Nonaccrual Loans	990	341
Commercial tax-exempt
Financing Receivable, Past Due [Line Items]
Accruing Past Due	0	0
Nonaccrual Loans	0	0
Current Accruing Loans	445,319	447,927
Total Loans Receivable	445,319	447,927
Commercial tax-exempt | 30-59 Days Past Due
Financing Receivable, Past Due [Line Items]
Accruing Past Due	0	0
Commercial tax-exempt | 60-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Accruing Past Due	0	0
Commercial tax-exempt | Current
Financing Receivable, Past Due [Line Items]
Nonaccrual Loans	0	0
Commercial tax-exempt | 30-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Nonaccrual Loans	0	0
Commercial tax-exempt | 90 Days or Greater Past Due
Financing Receivable, Past Due [Line Items]
Nonaccrual Loans	0	0
Commercial real estate
Financing Receivable, Past Due [Line Items]
Accruing Past Due	0	1,420
Nonaccrual Loans	5,392	0
Current Accruing Loans	2,620,907	2,549,854
Total Loans Receivable	2,626,299	2,551,274
Commercial real estate | 30-59 Days Past Due
Financing Receivable, Past Due [Line Items]
Accruing Past Due	0	1,420
Commercial real estate | 60-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Accruing Past Due	0	0
Commercial real estate | Current
Financing Receivable, Past Due [Line Items]
Nonaccrual Loans	5,392	0
Commercial real estate | 30-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Nonaccrual Loans	0	0
Commercial real estate | 90 Days or Greater Past Due
Financing Receivable, Past Due [Line Items]
Nonaccrual Loans	0	0
Construction and land
Financing Receivable, Past Due [Line Items]
Accruing Past Due	0	0
Nonaccrual Loans	0	0
Current Accruing Loans	238,293	225,983
Total Loans Receivable	238,293	225,983
Construction and land | 30-59 Days Past Due
Financing Receivable, Past Due [Line Items]
Accruing Past Due	0	0
Construction and land | 60-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Accruing Past Due	0	0
Construction and land | Current
Financing Receivable, Past Due [Line Items]
Nonaccrual Loans	0	0
Construction and land | 30-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Nonaccrual Loans	0	0
Construction and land | 90 Days or Greater Past Due
Financing Receivable, Past Due [Line Items]
Nonaccrual Loans	0	0
Residential
Financing Receivable, Past Due [Line Items]
Accruing Past Due	12,967	20,171
Nonaccrual Loans	16,074	13,993
Current Accruing Loans	2,812,885	2,804,991
Total Loans Receivable	2,841,926	2,839,155
Residential | 30-59 Days Past Due
Financing Receivable, Past Due [Line Items]
Accruing Past Due	12,967	19,133
Residential | 60-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Accruing Past Due	0	1,038
Residential | Current
Financing Receivable, Past Due [Line Items]
Nonaccrual Loans	9,024	9,593
Residential | 30-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Nonaccrual Loans	2,056	759
Residential | 90 Days or Greater Past Due
Financing Receivable, Past Due [Line Items]
Nonaccrual Loans	$ 4,994	$ 3,641
Number of loans | loan	0	0
Home equity
Financing Receivable, Past Due [Line Items]
Accruing Past Due	$ 319	$ 369
Nonaccrual Loans	1,476	1,525
Current Accruing Loans	87,555	81,763
Total Loans Receivable	89,350	83,657
Home equity | 30-59 Days Past Due
Financing Receivable, Past Due [Line Items]
Accruing Past Due	0	369
Home equity | 60-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Accruing Past Due	319	0
Home equity | Current
Financing Receivable, Past Due [Line Items]
Nonaccrual Loans	180	220
Home equity | 30-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Nonaccrual Loans	0	148
Home equity | 90 Days or Greater Past Due
Financing Receivable, Past Due [Line Items]
Nonaccrual Loans	1,296	1,157
Consumer and other
Financing Receivable, Past Due [Line Items]
Accruing Past Due	73	3,157
Nonaccrual Loans	1	3
Current Accruing Loans	131,333	131,514
Total Loans Receivable	131,407	134,674
Consumer and other | 30-59 Days Past Due
Financing Receivable, Past Due [Line Items]
Accruing Past Due	73	1,008
Consumer and other | 60-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Accruing Past Due	0	2,149
Consumer and other | Current
Financing Receivable, Past Due [Line Items]
Nonaccrual Loans	1	1
Consumer and other | 30-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Nonaccrual Loans	0	0
Consumer and other | 90 Days or Greater Past Due
Financing Receivable, Past Due [Line Items]
Nonaccrual Loans	$ 0	$ 2