Loan Portfolio and Credit Quality Loans by Grade or Nonaccrual Status (Details) - USD ($) $ in Thousands		Jun. 30, 2018	Dec. 31, 2017
Financing Receivable, Recorded Investment [Line Items]
Total loans		$ 6,767,123	$ 6,505,028
Pass [Member] | Performing Financial Instruments [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans		6,652,571	6,350,207
Special Mention [Member] | Performing Financial Instruments [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans		53,448	87,174
Substandard [Member] | Performing Financial Instruments [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	[1]	45,453	53,352
Substandard [Member] | Nonperforming Financing Receivable [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans		15,651	14,295
Commercial and industrial [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans		583,193	520,992
Commercial and industrial [Member] | Pass [Member] | Performing Financial Instruments [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans		567,334	496,395
Commercial and industrial [Member] | Special Mention [Member] | Performing Financial Instruments [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans		6,845	12,898
Commercial and industrial [Member] | Substandard [Member] | Performing Financial Instruments [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	[1]	7,602	10,951
Commercial and industrial [Member] | Substandard [Member] | Nonperforming Financing Receivable [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans		1,412	748
Commercial Tax Exempt [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans		438,882	418,698
Commercial Tax Exempt [Member] | Pass [Member] | Performing Financial Instruments [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans		438,882	413,139
Commercial Tax Exempt [Member] | Special Mention [Member] | Performing Financial Instruments [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans		0	5,559
Commercial Tax Exempt [Member] | Substandard [Member] | Performing Financial Instruments [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	[1]	0	0
Commercial Tax Exempt [Member] | Substandard [Member] | Nonperforming Financing Receivable [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans		0	0
Commercial real estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans		2,504,521	2,440,220
Commercial real estate [Member] | Pass [Member] | Performing Financial Instruments [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans		2,426,821	2,346,833
Commercial real estate [Member] | Special Mention [Member] | Performing Financial Instruments [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans		46,603	56,947
Commercial real estate [Member] | Substandard [Member] | Performing Financial Instruments [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	[1]	29,259	34,455
Commercial real estate [Member] | Substandard [Member] | Nonperforming Financing Receivable [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans		1,838	1,985
Construction and land [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans		172,024	164,990
Construction and land [Member] | Pass [Member] | Performing Financial Instruments [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans		164,760	146,514
Construction and land [Member] | Special Mention [Member] | Performing Financial Instruments [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans		0	11,770
Construction and land [Member] | Substandard [Member] | Performing Financial Instruments [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	[1]	7,264	6,596
Construction and land [Member] | Substandard [Member] | Nonperforming Financing Receivable [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans		0	110
Residential [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans		2,808,206	2,682,533
Residential [Member] | Pass [Member] | Performing Financial Instruments [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans		2,797,270	2,672,714
Residential [Member] | Special Mention [Member] | Performing Financial Instruments [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans		0	0
Residential [Member] | Substandard [Member] | Performing Financial Instruments [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	[1]	1,326	1,349
Residential [Member] | Substandard [Member] | Nonperforming Financing Receivable [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans		9,610	8,470
Home equity [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans		91,801	99,958
Home equity [Member] | Pass [Member] | Performing Financial Instruments [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans		89,012	97,118
Home equity [Member] | Special Mention [Member] | Performing Financial Instruments [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans		0	0
Home equity [Member] | Substandard [Member] | Performing Financial Instruments [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	[1]	0	0
Home equity [Member] | Substandard [Member] | Nonperforming Financing Receivable [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans		2,789	2,840
Consumer and other [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans		168,496	177,637
Consumer and other [Member] | Pass [Member] | Performing Financial Instruments [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans		168,492	177,494
Consumer and other [Member] | Special Mention [Member] | Performing Financial Instruments [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans		0	0
Consumer and other [Member] | Substandard [Member] | Performing Financial Instruments [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	[1]	2	1
Consumer and other [Member] | Substandard [Member] | Nonperforming Financing Receivable [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans		$ 2	$ 142

[1] Accruing Classified includes both Substandard and Doubtful classifications.