Loan Portfolio and Credit Quality Loans by Grade or Nonaccrual Status (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Financing Receivable, Recorded Investment [Line Items]
Total loans	$ 6,926,968	$ 6,893,158
Pass | Performing Financial Instruments
Financing Receivable, Recorded Investment [Line Items]
Total loans	6,785,641	6,746,478
Special Mention | Performing Financial Instruments
Financing Receivable, Recorded Investment [Line Items]
Total loans	86,787	78,528
Substandard | Performing Financial Instruments
Financing Receivable, Recorded Investment [Line Items]
Total loans	42,521	54,095
Substandard | Nonaccrual Loans
Financing Receivable, Recorded Investment [Line Items]
Total loans	12,019	14,057
Commercial and industrial
Financing Receivable, Recorded Investment [Line Items]
Total loans	615,370	623,037
Commercial and industrial | Pass | Performing Financial Instruments
Financing Receivable, Recorded Investment [Line Items]
Total loans	573,749	581,278
Commercial and industrial | Special Mention | Performing Financial Instruments
Financing Receivable, Recorded Investment [Line Items]
Total loans	17,143	16,213
Commercial and industrial | Substandard | Performing Financial Instruments
Financing Receivable, Recorded Investment [Line Items]
Total loans	22,986	22,992
Commercial and industrial | Substandard | Nonaccrual Loans
Financing Receivable, Recorded Investment [Line Items]
Total loans	1,492	2,554
Commercial tax-exempt
Financing Receivable, Recorded Investment [Line Items]
Total loans	449,492	451,671
Commercial tax-exempt | Pass | Performing Financial Instruments
Financing Receivable, Recorded Investment [Line Items]
Total loans	442,708	444,835
Commercial tax-exempt | Special Mention | Performing Financial Instruments
Financing Receivable, Recorded Investment [Line Items]
Total loans	2,732	2,785
Commercial tax-exempt | Substandard | Performing Financial Instruments
Financing Receivable, Recorded Investment [Line Items]
Total loans	4,052	4,051
Commercial tax-exempt | Substandard | Nonaccrual Loans
Financing Receivable, Recorded Investment [Line Items]
Total loans	0	0
Commercial real estate
Financing Receivable, Recorded Investment [Line Items]
Total loans	2,439,048	2,395,692
Commercial real estate | Pass | Performing Financial Instruments
Financing Receivable, Recorded Investment [Line Items]
Total loans	2,366,113	2,314,223
Commercial real estate | Special Mention | Performing Financial Instruments
Financing Receivable, Recorded Investment [Line Items]
Total loans	60,452	53,871
Commercial real estate | Substandard | Performing Financial Instruments
Financing Receivable, Recorded Investment [Line Items]
Total loans	12,483	27,052
Commercial real estate | Substandard | Nonaccrual Loans
Financing Receivable, Recorded Investment [Line Items]
Total loans	0	546
Construction and land
Financing Receivable, Recorded Investment [Line Items]
Total loans	210,618	240,306
Construction and land | Pass | Performing Financial Instruments
Financing Receivable, Recorded Investment [Line Items]
Total loans	204,158	234,647
Construction and land | Special Mention | Performing Financial Instruments
Financing Receivable, Recorded Investment [Line Items]
Total loans	6,460	5,659
Construction and land | Substandard | Performing Financial Instruments
Financing Receivable, Recorded Investment [Line Items]
Total loans	0	0
Construction and land | Substandard | Nonaccrual Loans
Financing Receivable, Recorded Investment [Line Items]
Total loans	0	0
Residential
Financing Receivable, Recorded Investment [Line Items]
Total loans	2,993,132	2,948,973
Residential | Pass | Performing Financial Instruments
Financing Receivable, Recorded Investment [Line Items]
Total loans	2,982,281	2,941,059
Residential | Special Mention | Performing Financial Instruments
Financing Receivable, Recorded Investment [Line Items]
Total loans	0	0
Residential | Substandard | Performing Financial Instruments
Financing Receivable, Recorded Investment [Line Items]
Total loans	3,000	0
Residential | Substandard | Nonaccrual Loans
Financing Receivable, Recorded Investment [Line Items]
Total loans	7,851	7,914
Home equity
Financing Receivable, Recorded Investment [Line Items]
Total loans	88,620	90,421
Home equity | Pass | Performing Financial Instruments
Financing Receivable, Recorded Investment [Line Items]
Total loans	85,952	87,390
Home equity | Special Mention | Performing Financial Instruments
Financing Receivable, Recorded Investment [Line Items]
Total loans	0	0
Home equity | Substandard | Performing Financial Instruments
Financing Receivable, Recorded Investment [Line Items]
Total loans	0	0
Home equity | Substandard | Nonaccrual Loans
Financing Receivable, Recorded Investment [Line Items]
Total loans	2,668	3,031
Consumer and other
Financing Receivable, Recorded Investment [Line Items]
Total loans	130,688	143,058
Consumer and other | Pass | Performing Financial Instruments
Financing Receivable, Recorded Investment [Line Items]
Total loans	130,680	143,046
Consumer and other | Special Mention | Performing Financial Instruments
Financing Receivable, Recorded Investment [Line Items]
Total loans	0	0
Consumer and other | Substandard | Performing Financial Instruments
Financing Receivable, Recorded Investment [Line Items]
Total loans	0	0
Consumer and other | Substandard | Nonaccrual Loans
Financing Receivable, Recorded Investment [Line Items]
Total loans	$ 8	$ 12