Loan Portfolio and Credit Quality Loans by Grade or Nonaccrual Status (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Financing Receivable, Recorded Investment [Line Items]
Total loans	$ 6,250,217	$ 6,114,354
Pass [Member] | Performing Financial Instruments [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	6,115,097	5,995,833
Special Mention [Member] | Performing Financial Instruments [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	49,796	37,535
Substandard [Member] | Performing Financial Instruments [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	64,379	63,671
Substandard [Member] | Nonperforming Financing Receivable [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	20,945	17,315
Commercial and industrial [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	580,354	611,370
Commercial and industrial [Member] | Pass [Member] | Performing Financial Instruments [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	558,621	591,388
Commercial and industrial [Member] | Special Mention [Member] | Performing Financial Instruments [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	10,743	10,133
Commercial and industrial [Member] | Substandard [Member] | Performing Financial Instruments [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	10,418	9,277
Commercial and industrial [Member] | Substandard [Member] | Nonperforming Financing Receivable [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	572	572
Commercial Tax Exempt [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	409,432	398,604
Commercial Tax Exempt [Member] | Pass [Member] | Performing Financial Instruments [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	399,442	388,544
Commercial Tax Exempt [Member] | Special Mention [Member] | Performing Financial Instruments [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	5,653	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	4,337	0
Commercial real estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	2,368,627	2,302,244
Commercial real estate [Member] | Pass [Member] | Performing Financial Instruments [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	2,282,055	2,230,732
Commercial real estate [Member] | Special Mention [Member] | Performing Financial Instruments [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	33,291	17,233
Commercial real estate [Member] | Substandard [Member] | Performing Financial Instruments [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	49,425	49,696
Commercial real estate [Member] | Substandard [Member] | Nonperforming Financing Receivable [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	3,856	4,583
Construction and land [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	117,007	104,839
Construction and land [Member] | Pass [Member] | Performing Financial Instruments [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	113,604	101,254
Construction and land [Member] | Special Mention [Member] | Performing Financial Instruments [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	109	109
Construction and land [Member] | Substandard [Member] | Performing Financial Instruments [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	3,147	3,297
Construction and land [Member] | Substandard [Member] | Nonperforming Financing Receivable [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	147	179
Residential [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	2,463,616	2,379,861
Residential [Member] | Pass [Member] | Performing Financial Instruments [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	2,451,267	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	1,387	1,399
Residential [Member] | Substandard [Member] | Nonperforming Financing Receivable [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	10,962	10,908
Home equity [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	114,536	118,817
Home equity [Member] | Pass [Member] | Performing Financial Instruments [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	113,466	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	196,645	198,619
Consumer and other [Member] | Pass [Member] | Performing Financial Instruments [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	196,642	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	2	2
Consumer and other [Member] | Substandard [Member] | Nonperforming Financing Receivable [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	$ 1	$ 1