Loan Portfolio and Credit Quality Loans by Past Due Status (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	$ 20,376	$ 25,048
Financing Receivable, Recorded Investment, 90 Days Past Due and Still Accruing	0	0
Financing Receivable, Recorded Investment, Nonaccrual Status	16,380	14,295
Financing Receivable, Recorded Investment, Current	6,565,571	6,465,685
Total loans	6,602,327	6,505,028
Financing Receivables, 1 to 29 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Nonaccrual Status	3,562	1,340
Financing Receivables, 30 to 89 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Nonaccrual Status	3,372	3,368
Financing Receivables, 30 to 59 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	19,337	22,564
Financing Receivables, 60 to 89 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	1,039	2,484
Financing Receivables, Equal to Greater than 90 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Nonaccrual Status	9,446	9,587
Commercial and industrial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	4,594	11,752
Financing Receivable, Recorded Investment, Nonaccrual Status	1,669	748
Financing Receivable, Recorded Investment, Current	524,830	508,492
Total loans	531,093	520,992
Commercial and industrial [Member] | Financing Receivables, 1 to 29 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Nonaccrual Status	311	355
Commercial and industrial [Member] | Financing Receivables, 30 to 89 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Nonaccrual Status	100	0
Commercial and industrial [Member] | Financing Receivables, 30 to 59 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	3,894	10,903
Commercial and industrial [Member] | Financing Receivables, 60 to 89 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	700	849
Commercial and industrial [Member] | Financing Receivables, Equal to Greater than 90 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Nonaccrual Status	1,258	393
Commercial Tax Exempt [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	0	0
Financing Receivable, Recorded Investment, Nonaccrual Status	0	0
Financing Receivable, Recorded Investment, Current	420,757	418,698
Total loans	420,757	418,698
Commercial Tax Exempt [Member] | Financing Receivables, 1 to 29 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Nonaccrual Status	0	0
Commercial Tax Exempt [Member] | Financing Receivables, 30 to 89 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Nonaccrual Status	0	0
Commercial Tax Exempt [Member] | Financing Receivables, 30 to 59 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	0	0
Commercial Tax Exempt [Member] | Financing Receivables, 60 to 89 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	0	0
Commercial Tax Exempt [Member] | Financing Receivables, Equal to Greater than 90 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Nonaccrual Status	0	0
Commercial real estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	2,507	4,043
Financing Receivable, Recorded Investment, Nonaccrual Status	1,839	1,985
Financing Receivable, Recorded Investment, Current	2,460,657	2,434,192
Total loans	2,465,003	2,440,220
Commercial real estate [Member] | Financing Receivables, 1 to 29 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Nonaccrual Status	1	163
Commercial real estate [Member] | Financing Receivables, 30 to 89 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Nonaccrual Status	151	0
Commercial real estate [Member] | Financing Receivables, 30 to 59 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	2,507	4,043
Commercial real estate [Member] | Financing Receivables, 60 to 89 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	0	0
Commercial real estate [Member] | Financing Receivables, Equal to Greater than 90 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Nonaccrual Status	1,687	1,822
Construction and land [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	64	0
Financing Receivable, Recorded Investment, Nonaccrual Status	109	110
Financing Receivable, Recorded Investment, Current	165,067	164,880
Total loans	165,240	164,990
Construction and land [Member] | Financing Receivables, 1 to 29 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Nonaccrual Status	0	0
Construction and land [Member] | Financing Receivables, 30 to 89 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Nonaccrual Status	0	0
Construction and land [Member] | Financing Receivables, 30 to 59 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	64	0
Construction and land [Member] | Financing Receivables, 60 to 89 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	0	0
Construction and land [Member] | Financing Receivables, Equal to Greater than 90 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Nonaccrual Status	109	110
Residential [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	12,489	8,874
Financing Receivable, Recorded Investment, Nonaccrual Status	9,932	8,470
Financing Receivable, Recorded Investment, Current	2,714,948	2,665,189
Total loans	2,737,369	2,682,533
Residential [Member] | Financing Receivables, 1 to 29 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Nonaccrual Status	3,183	805
Residential [Member] | Financing Receivables, 30 to 89 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Nonaccrual Status	3,115	3,172
Residential [Member] | Financing Receivables, 30 to 59 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	12,489	7,239
Residential [Member] | Financing Receivables, 60 to 89 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	0	1,635
Residential [Member] | Financing Receivables, Equal to Greater than 90 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Nonaccrual Status	3,634	4,493
Home equity [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	664	355
Financing Receivable, Recorded Investment, Nonaccrual Status	2,816	2,840
Financing Receivable, Recorded Investment, Current	90,851	96,763
Total loans	94,331	99,958
Home equity [Member] | Financing Receivables, 1 to 29 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Nonaccrual Status	67	0
Home equity [Member] | Financing Receivables, 30 to 89 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Nonaccrual Status	0	71
Home equity [Member] | Financing Receivables, 30 to 59 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	325	355
Home equity [Member] | Financing Receivables, 60 to 89 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	339	0
Home equity [Member] | Financing Receivables, Equal to Greater than 90 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Nonaccrual Status	2,749	2,769
Consumer and other [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	58	24
Financing Receivable, Recorded Investment, Nonaccrual Status	15	142
Financing Receivable, Recorded Investment, Current	188,461	177,471
Total loans	188,534	177,637
Consumer and other [Member] | Financing Receivables, 1 to 29 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Nonaccrual Status	0	17
Consumer and other [Member] | Financing Receivables, 30 to 89 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Nonaccrual Status	6	125
Consumer and other [Member] | Financing Receivables, 30 to 59 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	58	24
Consumer and other [Member] | Financing Receivables, 60 to 89 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	0	0
Consumer and other [Member] | Financing Receivables, Equal to Greater than 90 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Nonaccrual Status	$ 9	$ 0