Regulatory Matters (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Common equity tier 1 risk-based capital
Common equity tier 1 risk-based capital, actual, amount	$ 745,926	$ 702,728
Common equity tier 1 risk-based capital, actual, ratio	11.42%	11.40%
Common equity tier 1 risk-based capital, for capital adequacy purposes (at least), amount	$ 293,886	$ 277,275
Common equity tier 1 risk-based capital, for capital adequacy purposes (at least), ratio	4.50%	4.50%
Tier 1 risk-based capital
Tier 1 risk-based capital, actual, amount	$ 846,337	$ 803,311
Tier 1 risk-based capital, actual, ratio	12.96%	13.04%
Tier 1 risk-based capital, for capital adequacy purposes (at least), amount	$ 391,848	$ 369,701
Tier 1 risk-based capital, for capital adequacy purposes (at least), ratio	6.00%	6.00%
Total risk-based capital
Total risk-based capital, actual, amount	$ 919,573	$ 879,927
Total risk-based capital, actual, ratio	14.08%	14.28%
Total risk-based capital, for capital adequacy purposes (at least), amount	$ 522,464	$ 492,934
Total risk-based capital, for capital adequacy purposes (at least), ratio	8.00%	8.00%
Tier 1 leverage capital
Tier 1 leverage capital, actual, amount	$ 846,337	$ 803,311
Tier 1 leverage capital, actual, ratio	9.77%	9.54%
Tier 1 leverage capital, for capital adequacy purposes (at least), amount	$ 346,398	$ 336,648
Tier 1 leverage capital, for capital adequacy purposes (at least), ratio	4.00%	4.00%
Basel III Minimum with Capital Conservation Buffer [Domain]
Common equity tier 1 risk-based capital
Common equity tier 1 risk-based capital, for capital adequacy purposes (at least), ratio	7.00%	7.00%
Tier 1 risk-based capital
Tier 1 risk-based capital, for capital adequacy purposes (at least), ratio	8.50%	8.50%
Total risk-based capital
Total risk-based capital, for capital adequacy purposes (at least), ratio	10.50%	10.50%
Tier 1 leverage capital
Tier 1 leverage capital, for capital adequacy purposes (at least), ratio	4.00%	4.00%
Boston Private Bank
Common equity tier 1 risk-based capital
Common equity tier 1 risk-based capital, actual, amount	$ 778,635	$ 745,051
Common equity tier 1 risk-based capital, actual, ratio	11.97%	12.13%
Common equity tier 1 risk-based capital, for capital adequacy purposes (at least), amount	$ 292,717	$ 276,352
Common equity tier 1 risk-based capital, for capital adequacy purposes (at least), ratio	4.50%	4.50%
Common equity tier 1 risk-based capital, to be well capitalized under prompt corrective action provisions (at least), amount	$ 422,813	$ 399,175
Common equity tier 1 risk-based capital, to be well capitalized under prompt corrective action provisions (at least), ratio	6.50%	6.50%
Tier 1 risk-based capital
Tier 1 risk-based capital, actual, amount	$ 778,635	$ 745,051
Tier 1 risk-based capital, actual, ratio	11.97%	12.13%
Tier 1 risk-based capital, for capital adequacy purposes (at least), amount	$ 390,289	$ 368,469
Tier 1 risk-based capital, for capital adequacy purposes (at least), ratio	6.00%	6.00%
Tier 1 risk-based capital, to be well capitalized under prompt corrective action provisions (at least), amount	$ 520,386	$ 491,292
Tier 1 risk-based capital, to be well capitalized under prompt corrective action provisions (at least), ratio	8.00%	8.00%
Total risk-based capital
Total risk-based capital, actual, amount	$ 851,733	$ 821,584
Total risk-based capital, actual, ratio	13.09%	13.38%
Total risk-based capital, for capital adequacy purposes (at least), amount	$ 520,386	$ 491,292
Total risk-based capital, for capital adequacy purposes (at least), ratio	8.00%	8.00%
Total risk-based capital, to be well capitalized under prompt corrective action provisions (at least), ratio	$ 650,482	$ 614,115
Total risk-based capital, to be well capitalized under prompt corrective action provisions (at least), amount	10.00%	10.00%
Tier 1 leverage capital
Tier 1 leverage capital, actual, amount	$ 778,635	$ 745,051
Tier 1 leverage capital, actual, ratio	9.03%	8.92%
Tier 1 leverage capital, for capital adequacy purposes (at least), amount	$ 344,958	$ 334,029
Tier 1 leverage capital, for capital adequacy purposes (at least), ratio	4.00%	4.00%
Tier 1 leverage capital, to be well capitalized under prompt corrective action provisions (at least), amount	$ 431,198	$ 417,537
Tier 1 leverage capital, to be well capitalized under prompt corrective action provisions (at least), ratio	5.00%	5.00%
Boston Private Bank \| Basel III Minimum with Capital Conservation Buffer [Domain]
Common equity tier 1 risk-based capital
Common equity tier 1 risk-based capital, for capital adequacy purposes (at least), ratio	7.00%	7.00%
Tier 1 risk-based capital
Tier 1 risk-based capital, for capital adequacy purposes (at least), ratio	8.50%	8.50%
Total risk-based capital
Total risk-based capital, for capital adequacy purposes (at least), ratio	10.50%	10.50%
Tier 1 leverage capital
Tier 1 leverage capital, for capital adequacy purposes (at least), ratio	4.00%	4.00%

Common Equity Tier One Capital Required for Capital Adequacy to Risk Weighted Assets

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Common Equity Tier One Risk Based Capital Required to be Well Capitalized to Risk Weighted Assets

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The amount of total capital as defined in the regulations.

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The minimum Total risk based capital ratio (total capital divided by risk weighted assets) required for capital adequacy purposes under the regulatory framework for prompt corrective action.

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The amount of Total Capital required to be categorized as well as capitalized under the regulatory framework for prompt corrective action.

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The Total risk based capital ratio (total capital divided by risk weighted assets) required to be categorized as "well capitalized", in the regulatory framework for prompt corrective action.

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Total Capital divided by risk weighted assets as defined in the regulations.

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Amount of Common Equity Tier One Risk-based Capital, as defined by regulatory framework.

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Percentage of Common Equity Tier One Risk-based Capital to total risk-weighted assets, as defined by regulatory framework.

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Amount of Common Equity Tier 1 Risk-based Capital required to be categorized as adequately capitalized under regulatory framework for prompt corrective action.

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Amount of Common Equity Tier 1 Risk-based Capital required to be categorized as well-capitalized under regulatory framework for prompt corrective action.

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Tier 1 Leverage Capital as defined in the regulations. Also called Core Capital.

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The amount of Tier 1 Leverage Capital required to be categorized as well capitalized under the regulatory framework for prompt corrective action.

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The amount of Tier 1 Leverage Capital required to be categorized as well capitalized divided by average assets as defined in the regulatory framework for prompt corrective action.

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Tier 1 Leverage Capital divided by average assets as defined in the regulations.

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Tier 1 Risk Based Capital as defined in the regulations.

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Tier 1 capital divided by risk weighted assets as defined by regulations.

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