Loan Portfolio and Credit Quality Loans by Grade or Nonaccrual Status (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	$ 7,067,151	$ 6,893,158
Pass | Performing Financial Instruments
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	6,928,175	6,746,478
Special Mention | Performing Financial Instruments
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	58,133	78,528
Substandard | Performing Financial Instruments
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	63,278	54,095
Substandard | Nonaccrual Loans
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	17,565	14,057
Commercial and industrial
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	695,029	623,037
Commercial and industrial | Pass | Performing Financial Instruments
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	656,012	581,278
Commercial and industrial | Special Mention | Performing Financial Instruments
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	13,084	16,213
Commercial and industrial | Substandard | Performing Financial Instruments
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	25,133	22,992
Commercial and industrial | Substandard | Nonaccrual Loans
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	800	2,554
Commercial tax-exempt
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	448,488	451,671
Commercial tax-exempt | Pass | Performing Financial Instruments
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	441,811	444,835
Commercial tax-exempt | Special Mention | Performing Financial Instruments
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	2,625	2,785
Commercial tax-exempt | Substandard | Performing Financial Instruments
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	4,052	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	2,533,346	2,395,692
Commercial real estate | Pass | Performing Financial Instruments
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	2,460,408	2,314,223
Commercial real estate | Special Mention | Performing Financial Instruments
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	42,124	53,871
Commercial real estate | Substandard | Performing Financial Instruments
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	30,814	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	209,741	240,306
Construction and land | Pass | Performing Financial Instruments
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	209,741	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	2,964,042	2,948,973
Residential | Pass | Performing Financial Instruments
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	2,946,823	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	14,219	7,914
Home equity
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	84,432	90,421
Home equity | Pass | Performing Financial Instruments
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	81,308	87,390
Home equity | Special Mention | Performing Financial Instruments
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	300	0
Home equity | Substandard | Performing Financial Instruments
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	279	0
Home equity | Substandard | Nonaccrual Loans
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	2,545	3,031
Consumer and other
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	132,073	143,058
Consumer and other | Pass | Performing Financial Instruments
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	132,072	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	$ 1	$ 12