Acquisitions, Asset Sales, and Divestitures Acquisitions (Details) - USD ($) $ in Thousands, shares in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Business Acquisition [Line Items]
Goodwill, Acquired During Period	$ 0	$ 0
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Finite-Lived Intangibles	$ 0	$ 0
2014 Acquisition of Banyan Partners LLC [Member]
Business Acquisition [Line Items]
Business Acquisition, Name of Acquired Entity				Banyan Partners, LLC
Business Acquisition, Effective Date of Acquisition				Oct. 02, 2014
Business Acquisition, Description of Acquired Entity				Banyan, a registered investment advisory firm headquartered in Palm Beach Gardens, Florida. At the time of acquisition, Banyan had approximately $4.3 billion in client assets and locations in New England; South Florida; Texas; California; Atlanta, Georgia; and Madison, Wisconsin.
Business Acquisition, Percentage of Voting Interests Acquired				100.00%
Business Acquisition, Equity Interest Issued or Issuable, Number of Shares				1.7
Business Combination, Consideration Transferred, Equity Interests Issued and Issuable				$ 21,000
Business Combination, Consideration Transferred				43,900
Business Combination, Contingent Consideration, Asset				5,000
Business Combination, Contingent Consideration, Liability				2,000
Payments to Acquire Businesses, Gross				66,900
Business Combination, Contingent Consideration Arrangements, Change in Amount of Contingent Consideration, Liability			$ 2,000
Goodwill, Acquired During Period				41,900
Business Acquisition, Goodwill, Expected Tax Deductible Amount				39,900
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Finite-Lived Intangibles				$ 23,900
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life				10 years
Business Combination, Reason for Business Combination				The rationale for the transaction was that by merging Banyan with the existing Boston Private Bank wealth management business, additional technical expertise and financial acumen would be generated. Banyan has been merged with the existing wealth management business from Boston Private Bank, and the combined company has been renamed Boston Private Wealth. As a wholly-owned subsidiary of the Bank, Boston Private Wealth will also operate as part of the Wealth Management and Trust segment along with the trust operations of the Bank. These operations are reported separately from the Private Banking operations of the Bank.