Loans, Allowance for Loan Losses and Credit Quality (Age Analysis of Past Due Financing Receivables) (Details) $ in Thousands	Mar. 31, 2017 USD ($) loan	Dec. 31, 2016 USD ($) loan
Age Analysis of Past Due Financing Receivables
Number of Loans 30-59 Days | loan	285	301
Number of Loans 60-89 Days | loan	40	72
Number of Loans 90 Days or More | loan	114	91
Number of Loans Total Past Due | loan	439	464
Principal Balance Total Past Due	$ 35,071	$ 19,911
Current	6,029,295	5,979,694
Total loans	6,064,366	5,999,605
Recorded Investment >90 Days and Accruing	$ 2	$ 2
Commercial and Industrial [Member]
Age Analysis of Past Due Financing Receivables
Number of Loans 30-59 Days | loan	10	8
Number of Loans 60-89 Days | loan	6	32
Number of Loans 90 Days or More | loan	34	6
Number of Loans Total Past Due | loan	50	46
Principal Balance Total Past Due	$ 18,878	$ 2,833
Current	862,451	899,220
Total loans	881,329	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	15	5
Number of Loans 60-89 Days | loan	0	8
Number of Loans 90 Days or More | loan	9	8
Number of Loans Total Past Due | loan	24	21
Principal Balance Total Past Due	$ 6,299	$ 6,580
Current	3,021,006	3,004,218
Total loans	3,027,305	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	0	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	0	0
Principal Balance Total Past Due	$ 0	$ 0
Current	356,173	320,391
Total loans	356,173	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	9	9
Number of Loans 60-89 Days | loan	8	0
Number of Loans 90 Days or More | loan	13	19
Number of Loans Total Past Due | loan	30	28
Principal Balance Total Past Due	$ 559	$ 463
Current	125,815	122,263
Total loans	126,374	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	11	11
Number of Loans 60-89 Days | loan	6	9
Number of Loans 90 Days or More | loan	23	27
Number of Loans Total Past Due | loan	40	47
Principal Balance Total Past Due	$ 6,093	$ 6,734
Current	647,906	637,692
Total loans	653,999	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	14	19
Number of Loans 60-89 Days | loan	6	11
Number of Loans 90 Days or More | loan	18	16
Number of Loans Total Past Due | loan	38	46
Principal Balance Total Past Due	$ 3,033	$ 3,093
Current	1,005,738	985,054
Total loans	1,008,771	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	226	249
Number of Loans 60-89 Days | loan	14	12
Number of Loans 90 Days or More | loan	17	15
Number of Loans Total Past Due | loan	257	276
Principal Balance Total Past Due	$ 209	$ 208
Current	10,206	10,856
Total loans	10,415	11,064
Recorded Investment >90 Days and Accruing	2	2
Financing Receivables, 30 to 59 Days Past Due [Member]
Age Analysis of Past Due Financing Receivables
Principal Balance Total Past Due	9,752	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	3,089	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	3,158	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	0	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	331	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	1,867	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	1,156	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	151	184
Financing Receivables, 60 to 89 Days Past Due [Member]
Age Analysis of Past Due Financing Receivables
Principal Balance Total Past Due	16,244	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	14,666	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	108	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	968	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	460	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	42	17
Financing Receivables, Equal to Greater than 90 Days Past Due [Member]
Age Analysis of Past Due Financing Receivables
Principal Balance Total Past Due	9,075	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	1,123	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	3,141	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	120	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	3,258	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	1,417	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	$ 16	$ 7