Loans Receivable and Credit Quality Loans by Credit Quality Indicator (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Financing Receivable, Recorded Investment [Line Items]
Total loans	$ 6,505,028	$ 6,114,354
Commercial Loan [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	520,992	611,370
Commercial Tax Exempt [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	418,698	398,604
Commercial Real Estate Portfolio Segment [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	2,440,220	2,302,244
Construction Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	164,990	104,839
Residential Portfolio Segment [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	2,682,533	2,379,861
Home Equity Line of Credit [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	99,958	118,817
Consumer Portfolio Segment [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	177,637	198,619
Performing Financial Instruments [Member] | Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	6,350,207	5,995,833
Performing Financial Instruments [Member] | Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	87,174	37,535
Performing Financial Instruments [Member] | Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	53,352	63,671
Performing Financial Instruments [Member] | Commercial Loan [Member] | Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	496,395	591,388
Performing Financial Instruments [Member] | Commercial Loan [Member] | Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	12,898	10,133
Performing Financial Instruments [Member] | Commercial Loan [Member] | Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	10,951	9,277
Performing Financial Instruments [Member] | Commercial Tax Exempt [Member] | Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	413,139	388,544
Performing Financial Instruments [Member] | Commercial Tax Exempt [Member] | Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	5,559	10,060
Performing Financial Instruments [Member] | Commercial Tax Exempt [Member] | Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	0	0
Performing Financial Instruments [Member] | Commercial Real Estate Portfolio Segment [Member] | Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	2,346,833	2,230,732
Performing Financial Instruments [Member] | Commercial Real Estate Portfolio Segment [Member] | Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	56,947	17,233
Performing Financial Instruments [Member] | Commercial Real Estate Portfolio Segment [Member] | Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	34,455	49,696
Performing Financial Instruments [Member] | Construction Loans [Member] | Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	146,514	101,254
Performing Financial Instruments [Member] | Construction Loans [Member] | Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	11,770	109
Performing Financial Instruments [Member] | Construction Loans [Member] | Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	6,596	3,297
Performing Financial Instruments [Member] | Residential Portfolio Segment [Member] | Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	2,672,714	2,367,554
Performing Financial Instruments [Member] | Residential Portfolio Segment [Member] | Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	0	0
Performing Financial Instruments [Member] | Residential Portfolio Segment [Member] | Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	1,349	1,399
Performing Financial Instruments [Member] | Home Equity Line of Credit [Member] | Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	97,118	117,745
Performing Financial Instruments [Member] | Home Equity Line of Credit [Member] | Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	0	0
Performing Financial Instruments [Member] | Home Equity Line of Credit [Member] | Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	0	0
Performing Financial Instruments [Member] | Consumer Portfolio Segment [Member] | Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	177,494	198,616
Performing Financial Instruments [Member] | Consumer Portfolio Segment [Member] | Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	0	0
Performing Financial Instruments [Member] | Consumer Portfolio Segment [Member] | Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	1	2
Nonperforming Financial Instruments [Member] | Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	14,295	17,315
Nonperforming Financial Instruments [Member] | Commercial Loan [Member] | Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	748	572
Nonperforming Financial Instruments [Member] | Commercial Tax Exempt [Member] | Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	0	0
Nonperforming Financial Instruments [Member] | Commercial Real Estate Portfolio Segment [Member] | Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	1,985	4,583
Nonperforming Financial Instruments [Member] | Construction Loans [Member] | Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	110	179
Nonperforming Financial Instruments [Member] | Residential Portfolio Segment [Member] | Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	8,470	10,908
Nonperforming Financial Instruments [Member] | Home Equity Line of Credit [Member] | Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	2,840	1,072
Nonperforming Financial Instruments [Member] | Consumer Portfolio Segment [Member] | Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	$ 142	$ 1