Junior Subordinated Debentures Boston Private Capital Trust I Terms (Details) - USD ($)	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Debt interest rate terms	provided, however, that the interest rate does not exceed the highest rate permitted by New York law, and may be modified by the U.S. law of general application
Boston Private Capital Trust I Junior Subordinated Debentures
Debt Instrument [Line Items]
Debt face amount	$ 105,000,000
Liquidation price per share	$ 50.00
Frequency of periodic payment	quarterly
Debt interest rate terms	fixed
Debt effective interest rate	4.875%
Maturity date	Oct. 01, 2034
Assets Held-in-trust	$ 3,200,000	$ 3,200,000

Junior Subordinated Debt, Liquidation Price per Share

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The total amount of cash and securities held by third party trustees pursuant to terms of debt instruments or other agreements as of the date of each statement of financial position presented, which can be used by the trustee only to pay the noncurrent portion of specified obligations.

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Face (par) amount of debt instrument at time of issuance.

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Description of the frequency of periodic payments (monthly, quarterly, annual).

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Effective interest rate for the funds borrowed under the debt agreement considering interest compounding and original issue discount or premium.

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Description of the interest rate as being fixed or variable, and, if variable, identification of the index or rate on which the interest rate is based and the number of points or percentage added to that index or rate to set the rate, and other pertinent information, such as frequency of rate resets.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Date when the debt instrument is scheduled to be fully repaid, in CCYY-MM-DD format.

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