Regulatory Matters (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Common equity tier 1 risk-based capital
Common equity tier 1 risk-based capital, actual, amount	$ 702,728	$ 607,800
Common equity tier 1 risk-based capital, actual, ratio	11.40%	10.32%
Common equity tier 1 risk-based capital, for capital adequacy purposes (at least), amount	$ 277,275	$ 265,153
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	$ 803,311	$ 758,089
Tier 1 risk-based capital, actual, ratio	13.04%	12.87%
Tier 1 risk-based capital, for capital adequacy purposes (at least), amount	$ 369,701	$ 353,537
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	$ 879,927	$ 832,182
Total risk-based capital, actual, ratio	14.28%	14.12%
Total risk-based capital, for capital adequacy purposes (at least), amount	$ 492,934	$ 471,383
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	$ 803,311	$ 758,089
Tier 1 leverage capital, actual, ratio	9.54%	9.34%
Tier 1 leverage capital, for capital adequacy purposes (at least), amount	$ 336,648	$ 324,725
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	$ 745,051	$ 694,201
Common equity tier 1 risk-based capital, actual, ratio	12.13%	11.83%
Common equity tier 1 risk-based capital, for capital adequacy purposes (at least), amount	$ 276,352	$ 264,028
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	$ 399,175	$ 381,373
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	$ 745,051	$ 694,201
Tier 1 risk-based capital, actual, ratio	12.13%	11.83%
Tier 1 risk-based capital, for capital adequacy purposes (at least), amount	$ 368,469	$ 352,037
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	$ 491,292	$ 469,382
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	$ 821,584	$ 767,576
Total risk-based capital, actual, ratio	13.38%	13.08%
Total risk-based capital, for capital adequacy purposes (at least), amount	$ 491,292	$ 469,382
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	$ 614,115	$ 586,728
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	$ 745,051	$ 694,201
Tier 1 leverage capital, actual, ratio	8.92%	8.63%
Tier 1 leverage capital, for capital adequacy purposes (at least), amount	$ 334,029	$ 321,920
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	$ 417,537	$ 402,400
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 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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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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