Loan Portfolio and Credit Quality Loans by Past Due Status (Details) $ in Thousands	Sep. 30, 2019 USD ($) loan	Dec. 31, 2018 USD ($) loan
Financing Receivable, Past Due [Line Items]
Accruing Past Due	$ 4,179	$ 22,299
Nonaccrual Loans	17,565	14,057
Current Accruing Loans	7,045,407	6,856,802
Total Loans Receivable	7,067,151	6,893,158
30-59 Days Past Due
Financing Receivable, Past Due [Line Items]
Accruing Past Due	1,140	16,540
60-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Accruing Past Due	3,039	5,759
Current
Financing Receivable, Past Due [Line Items]
Nonaccrual Loans	10,159	3,626
30-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Nonaccrual Loans	487	768
90 Days or Greater Past Due
Financing Receivable, Past Due [Line Items]
Nonaccrual Loans	6,919	9,663
Commercial and industrial
Financing Receivable, Past Due [Line Items]
Accruing Past Due	3,048	9,794
Nonaccrual Loans	800	2,554
Current Accruing Loans	691,181	610,689
Total Loans Receivable	695,029	623,037
Commercial and industrial | 30-59 Days Past Due
Financing Receivable, Past Due [Line Items]
Accruing Past Due	554	9,794
Commercial and industrial | 60-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Accruing Past Due	2,494	0
Commercial and industrial | Current
Financing Receivable, Past Due [Line Items]
Nonaccrual Loans	83	979
Commercial and industrial | 30-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Nonaccrual Loans	186	0
Commercial and industrial | 90 Days or Greater Past Due
Financing Receivable, Past Due [Line Items]
Nonaccrual Loans	531	1,575
Commercial tax-exempt
Financing Receivable, Past Due [Line Items]
Accruing Past Due	0	0
Nonaccrual Loans	0	0
Current Accruing Loans	448,488	451,671
Total Loans Receivable	448,488	451,671
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	497	0
Nonaccrual Loans	0	546
Current Accruing Loans	2,532,849	2,395,146
Total Loans Receivable	2,533,346	2,395,692
Commercial real estate | 30-59 Days Past Due
Financing Receivable, Past Due [Line Items]
Accruing Past Due	497	0
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	0	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	546
Construction and land
Financing Receivable, Past Due [Line Items]
Accruing Past Due	0	0
Nonaccrual Loans	0	0
Current Accruing Loans	209,741	240,306
Total Loans Receivable	209,741	240,306
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	266	6,843
Nonaccrual Loans	14,219	7,914
Current Accruing Loans	2,949,557	2,934,216
Total Loans Receivable	2,964,042	2,948,973
Residential | 30-59 Days Past Due
Financing Receivable, Past Due [Line Items]
Accruing Past Due	0	6,477
Residential | 60-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Accruing Past Due	266	366
Residential | Current
Financing Receivable, Past Due [Line Items]
Nonaccrual Loans	9,084	2,639
Residential | 30-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Nonaccrual Loans	301	716
Residential | 90 Days or Greater Past Due
Financing Receivable, Past Due [Line Items]
Nonaccrual Loans	$ 4,834	$ 4,559
Number of loans | loan	0	0
Home equity
Financing Receivable, Past Due [Line Items]
Accruing Past Due	$ 353	$ 602
Nonaccrual Loans	2,545	3,031
Current Accruing Loans	81,534	86,788
Total Loans Receivable	84,432	90,421
Home equity | 30-59 Days Past Due
Financing Receivable, Past Due [Line Items]
Accruing Past Due	74	252
Home equity | 60-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Accruing Past Due	279	350
Home equity | Current
Financing Receivable, Past Due [Line Items]
Nonaccrual Loans	991	0
Home equity | 30-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Nonaccrual Loans	0	48
Home equity | 90 Days or Greater Past Due
Financing Receivable, Past Due [Line Items]
Nonaccrual Loans	1,554	2,983
Consumer and other
Financing Receivable, Past Due [Line Items]
Accruing Past Due	15	5,060
Nonaccrual Loans	1	12
Current Accruing Loans	132,057	137,986
Total Loans Receivable	132,073	143,058
Consumer and other | 30-59 Days Past Due
Financing Receivable, Past Due [Line Items]
Accruing Past Due	15	17
Consumer and other | 60-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Accruing Past Due	0	5,043
Consumer and other | Current
Financing Receivable, Past Due [Line Items]
Nonaccrual Loans	1	8
Consumer and other | 30-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Nonaccrual Loans	0	4
Consumer and other | 90 Days or Greater Past Due
Financing Receivable, Past Due [Line Items]
Nonaccrual Loans	$ 0	$ 0