Loan Portfolio and Credit Quality Loans by Grade or Nonaccrual Status (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Financing Receivable, Recorded Investment [Line Items]
Total loans	$ 6,279,928	$ 6,114,354
Pass [Member] | Performing Financial Instruments [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	6,150,390	5,995,833
Special Mention [Member] | Performing Financial Instruments [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	62,264	37,535
Substandard [Member] | Performing Financial Instruments [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	51,098	63,671
Substandard [Member] | Nonperforming Financing Receivable [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	16,176	17,315
Commercial and industrial [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	541,756	611,370
Commercial and industrial [Member] | Pass [Member] | Performing Financial Instruments [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	522,349	591,388
Commercial and industrial [Member] | Special Mention [Member] | Performing Financial Instruments [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	12,135	10,133
Commercial and industrial [Member] | Substandard [Member] | Performing Financial Instruments [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	6,575	9,277
Commercial and industrial [Member] | Substandard [Member] | Nonperforming Financing Receivable [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	697	572
Commercial Tax Exempt [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	416,157	398,604
Commercial Tax Exempt [Member] | Pass [Member] | Performing Financial Instruments [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	410,535	388,544
Commercial Tax Exempt [Member] | Special Mention [Member] | Performing Financial Instruments [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	5,622	10,060
Commercial Tax Exempt [Member] | Substandard [Member] | Performing Financial Instruments [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	0	0
Commercial Tax Exempt [Member] | Substandard [Member] | Nonperforming Financing Receivable [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	0	0
Commercial real estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	2,356,345	2,302,244
Commercial real estate [Member] | Pass [Member] | Performing Financial Instruments [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	2,273,184	2,230,732
Commercial real estate [Member] | Special Mention [Member] | Performing Financial Instruments [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	40,177	17,233
Commercial real estate [Member] | Substandard [Member] | Performing Financial Instruments [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	39,980	49,696
Commercial real estate [Member] | Substandard [Member] | Nonperforming Financing Receivable [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	3,004	4,583
Construction and land [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	130,904	104,839
Construction and land [Member] | Pass [Member] | Performing Financial Instruments [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	123,420	101,254
Construction and land [Member] | Special Mention [Member] | Performing Financial Instruments [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	4,330	109
Construction and land [Member] | Substandard [Member] | Performing Financial Instruments [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	2,922	3,297
Construction and land [Member] | Substandard [Member] | Nonperforming Financing Receivable [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	232	179
Residential [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	2,525,225	2,379,861
Residential [Member] | Pass [Member] | Performing Financial Instruments [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	2,512,682	2,367,554
Residential [Member] | Special Mention [Member] | Performing Financial Instruments [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	0	0
Residential [Member] | Substandard [Member] | Performing Financial Instruments [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	1,370	1,399
Residential [Member] | Substandard [Member] | Nonperforming Financing Receivable [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	11,173	10,908
Home equity [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	108,549	118,817
Home equity [Member] | Pass [Member] | Performing Financial Instruments [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	107,479	117,745
Home equity [Member] | Special Mention [Member] | Performing Financial Instruments [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	0	0
Home equity [Member] | Substandard [Member] | Performing Financial Instruments [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	0	0
Home equity [Member] | Substandard [Member] | Nonperforming Financing Receivable [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	1,070	1,072
Consumer and other [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	200,992	198,619
Consumer and other [Member] | Pass [Member] | Performing Financial Instruments [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	200,741	198,616
Consumer and other [Member] | Special Mention [Member] | Performing Financial Instruments [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	0	0
Consumer and other [Member] | Substandard [Member] | Performing Financial Instruments [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	251	2
Consumer and other [Member] | Substandard [Member] | Nonperforming Financing Receivable [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	$ 0	$ 1