Loan Portfolio and Credit Quality Loans by Credit Quality Indicator (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Financing Receivable, Recorded Investment [Line Items]
Total loans	$ 6,893,158	$ 6,505,028
Commercial and industrial
Financing Receivable, Recorded Investment [Line Items]
Total loans	623,037	520,992
Commercial tax-exempt
Financing Receivable, Recorded Investment [Line Items]
Total loans	451,671	418,698
Commercial real estate
Financing Receivable, Recorded Investment [Line Items]
Total loans	2,395,692	2,440,220
Construction and land
Financing Receivable, Recorded Investment [Line Items]
Total loans	240,306	164,990
Residential
Financing Receivable, Recorded Investment [Line Items]
Total loans	2,948,973	2,682,533
Home equity
Financing Receivable, Recorded Investment [Line Items]
Total loans	90,421	99,958
Consumer and other
Financing Receivable, Recorded Investment [Line Items]
Total loans	143,058	177,637
Performing Financial Instruments [Member] | Pass
Financing Receivable, Recorded Investment [Line Items]
Total loans	6,746,478	6,350,207
Performing Financial Instruments [Member] | Special Mention
Financing Receivable, Recorded Investment [Line Items]
Total loans	78,528	87,174
Performing Financial Instruments [Member] | Substandard
Financing Receivable, Recorded Investment [Line Items]
Total loans	54,095	53,352
Performing Financial Instruments [Member] | Commercial and industrial | Pass
Financing Receivable, Recorded Investment [Line Items]
Total loans	581,278	496,395
Performing Financial Instruments [Member] | Commercial and industrial | Special Mention
Financing Receivable, Recorded Investment [Line Items]
Total loans	16,213	12,898
Performing Financial Instruments [Member] | Commercial and industrial | Substandard
Financing Receivable, Recorded Investment [Line Items]
Total loans	22,992	10,951
Performing Financial Instruments [Member] | Commercial tax-exempt | Pass
Financing Receivable, Recorded Investment [Line Items]
Total loans	444,835	413,139
Performing Financial Instruments [Member] | Commercial tax-exempt | Special Mention
Financing Receivable, Recorded Investment [Line Items]
Total loans	2,785	5,559
Performing Financial Instruments [Member] | Commercial tax-exempt | Substandard
Financing Receivable, Recorded Investment [Line Items]
Total loans	4,051	0
Performing Financial Instruments [Member] | Commercial real estate | Pass
Financing Receivable, Recorded Investment [Line Items]
Total loans	2,314,223	2,346,833
Performing Financial Instruments [Member] | Commercial real estate | Special Mention
Financing Receivable, Recorded Investment [Line Items]
Total loans	53,871	56,947
Performing Financial Instruments [Member] | Commercial real estate | Substandard
Financing Receivable, Recorded Investment [Line Items]
Total loans	27,052	34,455
Performing Financial Instruments [Member] | Construction and land | Pass
Financing Receivable, Recorded Investment [Line Items]
Total loans	234,647	146,514
Performing Financial Instruments [Member] | Construction and land | Special Mention
Financing Receivable, Recorded Investment [Line Items]
Total loans	5,659	11,770
Performing Financial Instruments [Member] | Construction and land | Substandard
Financing Receivable, Recorded Investment [Line Items]
Total loans	0	6,596
Performing Financial Instruments [Member] | Residential | Pass
Financing Receivable, Recorded Investment [Line Items]
Total loans	2,941,059	2,672,714
Performing Financial Instruments [Member] | Residential | Special Mention
Financing Receivable, Recorded Investment [Line Items]
Total loans	0	0
Performing Financial Instruments [Member] | Residential | Substandard
Financing Receivable, Recorded Investment [Line Items]
Total loans	0	1,349
Performing Financial Instruments [Member] | Home equity | Pass
Financing Receivable, Recorded Investment [Line Items]
Total loans	87,390	97,118
Performing Financial Instruments [Member] | Home equity | Special Mention
Financing Receivable, Recorded Investment [Line Items]
Total loans	0	0
Performing Financial Instruments [Member] | Home equity | Substandard
Financing Receivable, Recorded Investment [Line Items]
Total loans	0	0
Performing Financial Instruments [Member] | Consumer and other | Pass
Financing Receivable, Recorded Investment [Line Items]
Total loans	143,046	177,494
Performing Financial Instruments [Member] | Consumer and other | Special Mention
Financing Receivable, Recorded Investment [Line Items]
Total loans	0	0
Performing Financial Instruments [Member] | Consumer and other | Substandard
Financing Receivable, Recorded Investment [Line Items]
Total loans	0	1
Nonperforming Financial Instruments [Member] | Substandard
Financing Receivable, Recorded Investment [Line Items]
Total loans	14,057	14,295
Nonperforming Financial Instruments [Member] | Commercial and industrial | Substandard
Financing Receivable, Recorded Investment [Line Items]
Total loans	2,554	748
Nonperforming Financial Instruments [Member] | Commercial tax-exempt | Substandard
Financing Receivable, Recorded Investment [Line Items]
Total loans	0	0
Nonperforming Financial Instruments [Member] | Commercial real estate | Substandard
Financing Receivable, Recorded Investment [Line Items]
Total loans	546	1,985
Nonperforming Financial Instruments [Member] | Construction and land | Substandard
Financing Receivable, Recorded Investment [Line Items]
Total loans	0	110
Nonperforming Financial Instruments [Member] | Residential | Substandard
Financing Receivable, Recorded Investment [Line Items]
Total loans	7,914	8,470
Nonperforming Financial Instruments [Member] | Home equity | Substandard
Financing Receivable, Recorded Investment [Line Items]
Total loans	3,031	2,840
Nonperforming Financial Instruments [Member] | Consumer and other | Substandard
Financing Receivable, Recorded Investment [Line Items]
Total loans	$ 12	$ 142