Loan Portfolio and Credit Quality Loans by Grade or Nonaccrual Status (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	$ 7,080,260	$ 6,893,158
Pass | Performing Financial Instruments
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	6,938,545	6,746,478
Special Mention | Performing Financial Instruments
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	70,677	78,528
Substandard | Performing Financial Instruments
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	53,883	54,095
Substandard | Nonaccrual Loans
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	17,155	14,057
Commercial and industrial
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	656,186	623,037
Commercial and industrial | Pass | Performing Financial Instruments
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	617,524	581,278
Commercial and industrial | Special Mention | Performing Financial Instruments
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	14,058	16,213
Commercial and industrial | Substandard | Performing Financial Instruments
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	23,037	22,992
Commercial and industrial | Substandard | Nonaccrual Loans
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	1,567	2,554
Commercial tax-exempt
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	450,307	451,671
Commercial tax-exempt | Pass | Performing Financial Instruments
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	443,577	444,835
Commercial tax-exempt | Special Mention | Performing Financial Instruments
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	2,679	2,785
Commercial tax-exempt | Substandard | Performing Financial Instruments
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	4,051	4,051
Commercial tax-exempt | Substandard | Nonaccrual Loans
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	0	0
Commercial real estate
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	2,530,556	2,395,692
Commercial real estate | Pass | Performing Financial Instruments
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	2,452,821	2,314,223
Commercial real estate | Special Mention | Performing Financial Instruments
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	53,940	53,871
Commercial real estate | Substandard | Performing Financial Instruments
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	23,795	27,052
Commercial real estate | Substandard | Nonaccrual Loans
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	0	546
Construction and land
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	200,378	240,306
Construction and land | Pass | Performing Financial Instruments
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	200,378	234,647
Construction and land | Special Mention | Performing Financial Instruments
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	0	5,659
Construction and land | Substandard | Performing Financial Instruments
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	0	0
Construction and land | Substandard | Nonaccrual Loans
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	0	0
Residential
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	3,025,758	2,948,973
Residential | Pass | Performing Financial Instruments
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	3,010,186	2,941,059
Residential | Special Mention | Performing Financial Instruments
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	0	0
Residential | Substandard | Performing Financial Instruments
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	3,000	0
Residential | Substandard | Nonaccrual Loans
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	12,572	7,914
Home equity
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	89,930	90,421
Home equity | Pass | Performing Financial Instruments
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	86,926	87,390
Home equity | Special Mention | Performing Financial Instruments
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	0	0
Home equity | Substandard | Performing Financial Instruments
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	0	0
Home equity | Substandard | Nonaccrual Loans
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	3,004	3,031
Consumer and other
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	127,145	143,058
Consumer and other | Pass | Performing Financial Instruments
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	127,133	143,046
Consumer and other | Special Mention | Performing Financial Instruments
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	0	0
Consumer and other | Substandard | Performing Financial Instruments
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	0	0
Consumer and other | Substandard | Nonaccrual Loans
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	$ 12	$ 12