Loan Portfolio and Credit Quality Loans by Past Due Status (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Past Due	$ 17,715	$ 22,299
Nonaccrual Loans	12,019	14,057
Current Accruing Loans	6,897,234	6,856,802
Total Loans Receivable	6,926,968	6,893,158
30-59 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Past Due	14,033	16,540
60-89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Past Due	3,682	5,759
Current
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual Loans	5,907	3,626
30-89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual Loans	804	768
90 Days or Greater Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual Loans	5,308	9,663
Commercial and industrial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Past Due	1,692	9,794
Nonaccrual Loans	1,492	2,554
Current Accruing Loans	612,186	610,689
Total Loans Receivable	615,370	623,037
Commercial and industrial | 30-59 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Past Due	1,627	9,794
Commercial and industrial | 60-89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Past Due	65	0
Commercial and industrial | Current
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual Loans	1,274	979
Commercial and industrial | 30-89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual Loans	0	0
Commercial and industrial | 90 Days or Greater Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual Loans	218	1,575
Commercial tax-exempt
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Past Due	0	0
Nonaccrual Loans	0	0
Current Accruing Loans	449,492	451,671
Total Loans Receivable	449,492	451,671
Commercial tax-exempt | 30-59 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Past Due	0	0
Commercial tax-exempt | 60-89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Past Due	0	0
Commercial tax-exempt | Current
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual Loans	0	0
Commercial tax-exempt | 30-89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual Loans	0	0
Commercial tax-exempt | 90 Days or Greater Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual Loans	0	0
Commercial real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Past Due	2,700	0
Nonaccrual Loans	0	546
Current Accruing Loans	2,436,348	2,395,146
Total Loans Receivable	2,439,048	2,395,692
Commercial real estate | 30-59 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Past Due	0	0
Commercial real estate | 60-89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Past Due	2,700	0
Commercial real estate | Current
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual Loans	0	0
Commercial real estate | 30-89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual Loans	0	0
Commercial real estate | 90 Days or Greater Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual Loans	0	546
Construction and land
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Past Due	0	0
Nonaccrual Loans	0	0
Current Accruing Loans	210,618	240,306
Total Loans Receivable	210,618	240,306
Construction and land | 30-59 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Past Due	0	0
Construction and land | 60-89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Past Due	0	0
Construction and land | Current
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual Loans	0	0
Construction and land | 30-89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual Loans	0	0
Construction and land | 90 Days or Greater Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual Loans	0	0
Residential
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Past Due	12,741	6,843
Nonaccrual Loans	7,851	7,914
Current Accruing Loans	2,972,540	2,934,216
Total Loans Receivable	2,993,132	2,948,973
Residential | 30-59 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Past Due	12,163	6,477
Residential | 60-89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Past Due	578	366
Residential | Current
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual Loans	4,438	2,639
Residential | 30-89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual Loans	804	716
Residential | 90 Days or Greater Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual Loans	2,609	4,559
Home equity
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Past Due	568	602
Nonaccrual Loans	2,668	3,031
Current Accruing Loans	85,384	86,788
Total Loans Receivable	88,620	90,421
Home equity | 30-59 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Past Due	229	252
Home equity | 60-89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Past Due	339	350
Home equity | Current
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual Loans	188	0
Home equity | 30-89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual Loans	0	48
Home equity | 90 Days or Greater Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual Loans	2,480	2,983
Consumer and other
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Past Due	14	5,060
Nonaccrual Loans	8	12
Current Accruing Loans	130,666	137,986
Total Loans Receivable	130,688	143,058
Consumer and other | 30-59 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Past Due	14	17
Consumer and other | 60-89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Past Due	0	5,043
Consumer and other | Current
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual Loans	7	8
Consumer and other | 30-89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual Loans	0	4
Consumer and other | 90 Days or Greater Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual Loans	$ 1	$ 0