Loans, Allowance for Loan Losses and Credit Quality (Age Analysis of Past Due Financing Receivables) (Details) $ in Thousands	Sep. 30, 2017 USD ($) loan	Dec. 31, 2016 USD ($) loan
Age Analysis of Past Due Financing Receivables
Number of Loans 30-59 Days | loan	295	301
Number of Loans 60-89 Days | loan	36	72
Number of Loans 90 Days or More | loan	122	91
Number of Loans Total Past Due | loan	453	464
Principal Balance Total Past Due	$ 51,358	$ 19,911
Current	6,238,544	5,979,694
Total loans	6,289,902	5,999,605
Recorded Investment >90 Days and Accruing	$ 7	$ 2
Commercial and Industrial [Member]
Age Analysis of Past Due Financing Receivables
Number of Loans 30-59 Days | loan	12	8
Number of Loans 60-89 Days | loan	2	32
Number of Loans 90 Days or More | loan	45	6
Number of Loans Total Past Due | loan	59	46
Principal Balance Total Past Due	$ 33,644	$ 2,833
Current	824,878	899,220
Total loans	858,522	902,053
Recorded Investment >90 Days and Accruing	$ 0	$ 0
Commercial Real Estate [Member]
Age Analysis of Past Due Financing Receivables
Number of Loans 30-59 Days | loan	10	5
Number of Loans 60-89 Days | loan	0	8
Number of Loans 90 Days or More | loan	8	8
Number of Loans Total Past Due | loan	18	21
Principal Balance Total Past Due	$ 6,168	$ 6,580
Current	3,080,992	3,004,218
Total loans	3,087,160	3,010,798
Recorded Investment >90 Days and Accruing	$ 0	$ 0
Construction Loans [Member]
Age Analysis of Past Due Financing Receivables
Number of Loans 30-59 Days | loan	1	0
Number of Loans 60-89 Days | loan	0	0
Number of Loans 90 Days or More | loan	0	0
Number of Loans Total Past Due | loan	1	0
Principal Balance Total Past Due	$ 566	$ 0
Current	394,701	320,391
Total loans	395,267	320,391
Recorded Investment >90 Days and Accruing	$ 0	$ 0
Small Business [Member]
Age Analysis of Past Due Financing Receivables
Number of Loans 30-59 Days | loan	11	9
Number of Loans 60-89 Days | loan	5	0
Number of Loans 90 Days or More | loan	17	19
Number of Loans Total Past Due | loan	33	28
Principal Balance Total Past Due	$ 251	$ 463
Current	130,405	122,263
Total loans	130,656	122,726
Recorded Investment >90 Days and Accruing	$ 0	$ 0
Residential Real Estate [Member]
Age Analysis of Past Due Financing Receivables
Number of Loans 30-59 Days | loan	12	11
Number of Loans 60-89 Days | loan	10	9
Number of Loans 90 Days or More | loan	20	27
Number of Loans Total Past Due | loan	42	47
Principal Balance Total Past Due	$ 6,360	$ 6,734
Current	749,770	637,692
Total loans	756,130	644,426
Recorded Investment >90 Days and Accruing	$ 0	$ 0
Consumer Home Equity [Member]
Age Analysis of Past Due Financing Receivables
Number of Loans 30-59 Days | loan	20	19
Number of Loans 60-89 Days | loan	9	11
Number of Loans 90 Days or More | loan	20	16
Number of Loans Total Past Due | loan	49	46
Principal Balance Total Past Due	$ 4,091	$ 3,093
Current	1,048,204	985,054
Total loans	1,052,295	988,147
Recorded Investment >90 Days and Accruing	$ 0	$ 0
Consumer - Other [Member]
Age Analysis of Past Due Financing Receivables
Number of Loans 30-59 Days | loan	229	249
Number of Loans 60-89 Days | loan	10	12
Number of Loans 90 Days or More | loan	12	15
Number of Loans Total Past Due | loan	251	276
Principal Balance Total Past Due	$ 278	$ 208
Current	9,594	10,856
Total loans	9,872	11,064
Recorded Investment >90 Days and Accruing	7	2
Financing Receivables, 30 to 59 Days Past Due [Member]
Age Analysis of Past Due Financing Receivables
Principal Balance Total Past Due	8,315	4,519
Financing Receivables, 30 to 59 Days Past Due [Member] | Commercial and Industrial [Member]
Age Analysis of Past Due Financing Receivables
Principal Balance Total Past Due	839	100
Financing Receivables, 30 to 59 Days Past Due [Member] | Commercial Real Estate [Member]
Age Analysis of Past Due Financing Receivables
Principal Balance Total Past Due	4,413	1,518
Financing Receivables, 30 to 59 Days Past Due [Member] | Construction Loans [Member]
Age Analysis of Past Due Financing Receivables
Principal Balance Total Past Due	566	0
Financing Receivables, 30 to 59 Days Past Due [Member] | Small Business [Member]
Age Analysis of Past Due Financing Receivables
Principal Balance Total Past Due	130	323
Financing Receivables, 30 to 59 Days Past Due [Member] | Residential Real Estate [Member]
Age Analysis of Past Due Financing Receivables
Principal Balance Total Past Due	1,125	1,277
Financing Receivables, 30 to 59 Days Past Due [Member] | Consumer Home Equity [Member]
Age Analysis of Past Due Financing Receivables
Principal Balance Total Past Due	1,016	1,117
Financing Receivables, 30 to 59 Days Past Due [Member] | Consumer - Other [Member]
Age Analysis of Past Due Financing Receivables
Principal Balance Total Past Due	226	184
Financing Receivables, 60 to 89 Days Past Due [Member]
Age Analysis of Past Due Financing Receivables
Principal Balance Total Past Due	3,275	4,944
Financing Receivables, 60 to 89 Days Past Due [Member] | Commercial and Industrial [Member]
Age Analysis of Past Due Financing Receivables
Principal Balance Total Past Due	315	253
Financing Receivables, 60 to 89 Days Past Due [Member] | Commercial Real Estate [Member]
Age Analysis of Past Due Financing Receivables
Principal Balance Total Past Due	0	1,957
Financing Receivables, 60 to 89 Days Past Due [Member] | Construction Loans [Member]
Age Analysis of Past Due Financing Receivables
Principal Balance Total Past Due	0	0
Financing Receivables, 60 to 89 Days Past Due [Member] | Small Business [Member]
Age Analysis of Past Due Financing Receivables
Principal Balance Total Past Due	14	0
Financing Receivables, 60 to 89 Days Past Due [Member] | Residential Real Estate [Member]
Age Analysis of Past Due Financing Receivables
Principal Balance Total Past Due	2,392	1,950
Financing Receivables, 60 to 89 Days Past Due [Member] | Consumer Home Equity [Member]
Age Analysis of Past Due Financing Receivables
Principal Balance Total Past Due	516	767
Financing Receivables, 60 to 89 Days Past Due [Member] | Consumer - Other [Member]
Age Analysis of Past Due Financing Receivables
Principal Balance Total Past Due	38	17
Financing Receivables, Equal to Greater than 90 Days Past Due [Member]
Age Analysis of Past Due Financing Receivables
Principal Balance Total Past Due	39,768	10,448
Financing Receivables, Equal to Greater than 90 Days Past Due [Member] | Commercial and Industrial [Member]
Age Analysis of Past Due Financing Receivables
Principal Balance Total Past Due	32,490	2,480
Financing Receivables, Equal to Greater than 90 Days Past Due [Member] | Commercial Real Estate [Member]
Age Analysis of Past Due Financing Receivables
Principal Balance Total Past Due	1,755	3,105
Financing Receivables, Equal to Greater than 90 Days Past Due [Member] | Construction Loans [Member]
Age Analysis of Past Due Financing Receivables
Principal Balance Total Past Due	0	0
Financing Receivables, Equal to Greater than 90 Days Past Due [Member] | Small Business [Member]
Age Analysis of Past Due Financing Receivables
Principal Balance Total Past Due	107	140
Financing Receivables, Equal to Greater than 90 Days Past Due [Member] | Residential Real Estate [Member]
Age Analysis of Past Due Financing Receivables
Principal Balance Total Past Due	2,843	3,507
Financing Receivables, Equal to Greater than 90 Days Past Due [Member] | Consumer Home Equity [Member]
Age Analysis of Past Due Financing Receivables
Principal Balance Total Past Due	2,559	1,209
Financing Receivables, Equal to Greater than 90 Days Past Due [Member] | Consumer - Other [Member]
Age Analysis of Past Due Financing Receivables
Principal Balance Total Past Due	$ 14	$ 7