REGULATORY MATTERS - Schedule of Risk Based Capital (Details) $ in Thousands	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)
Common equity tier 1 risk-based capital
Common equity tier 1 risk-based capital, actual, amount	$ 773,017	$ 745,926
Common equity tier 1 risk-based capital, actual, ratio	0.1153	0.1142
Common equity tier 1 risk-based capital, for capital adequacy purposes (at least), amount	$ 301,794	$ 293,886
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	$ 873,039	$ 846,337
Tier 1 risk-based capital, actual, ratio	0.1302	0.1296
Tier 1 risk-based capital, for capital adequacy purposes (at least), amount	$ 402,391	$ 391,848
Tier 1 risk-based capital, for capital adequacy purposes (at least), ratio	0.060	0.060
Total risk-based capital
Total risk-based capital, actual, amount	$ 956,919	$ 919,573
Total risk-based capital, actual, ratio	0.1427	0.1408
Total risk-based capital, for capital adequacy purposes (at least), amount	$ 536,522	$ 522,464
Total risk-based capital, for capital adequacy purposes (at least), ratio	0.080	0.080
Tier 1 leverage capital
Tier 1 leverage capital, actual, amount	$ 873,039	$ 846,337
Tier 1 leverage capital, actual, ratio	0.0892	0.0977
Tier 1 leverage capital, for capital adequacy purposes (at least), amount	$ 391,634	$ 346,398
Tier 1 leverage capital, for capital adequacy purposes (at least), ratio	0.040	0.040
Basel III Minimum with Capital Conservation Buffer
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	0.085	0.085
Total risk-based capital
Total risk-based capital, for capital adequacy purposes (at least), ratio	0.105	0.105
Tier 1 leverage capital
Tier 1 leverage capital, for capital adequacy purposes (at least), ratio	0.040	0.040
Boston Private Bank
Common equity tier 1 risk-based capital
Common equity tier 1 risk-based capital, actual, amount	$ 788,375	$ 778,635
Common equity tier 1 risk-based capital, actual, ratio	0.1179	0.1197
Common equity tier 1 risk-based capital, for capital adequacy purposes (at least), amount	$ 301,003	$ 292,717
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	$ 434,782	$ 422,813
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	$ 788,375	$ 778,635
Tier 1 risk-based capital, actual, ratio	0.1179	0.1197
Tier 1 risk-based capital, for capital adequacy purposes (at least), amount	$ 401,337	$ 390,289
Tier 1 risk-based capital, for capital adequacy purposes (at least), ratio	0.060	0.060
Tier 1 risk-based capital, to be well capitalized under prompt corrective action provisions (at least), amount	$ 535,116	$ 520,386
Tier 1 risk-based capital, to be well capitalized under prompt corrective action provisions (at least), ratio	0.080	0.080
Total risk-based capital
Total risk-based capital, actual, amount	$ 872,038	$ 851,733
Total risk-based capital, actual, ratio	0.1304	0.1309
Total risk-based capital, for capital adequacy purposes (at least), amount	$ 535,116	$ 520,386
Total risk-based capital, for capital adequacy purposes (at least), ratio	0.080	0.080
Total risk-based capital, to be well capitalized under prompt corrective action provisions (at least), ratio	$ 668,896	$ 650,482
Total risk-based capital, to be well capitalized under prompt corrective action provisions (at least), amount	0.100	0.100
Tier 1 leverage capital
Tier 1 leverage capital, actual, amount	$ 788,375	$ 778,635
Tier 1 leverage capital, actual, ratio	0.0808	0.0903
Tier 1 leverage capital, for capital adequacy purposes (at least), amount	$ 390,364	$ 344,958
Tier 1 leverage capital, for capital adequacy purposes (at least), ratio	0.040	0.040
Tier 1 leverage capital, to be well capitalized under prompt corrective action provisions (at least), amount	$ 487,955	$ 431,198
Tier 1 leverage capital, to be well capitalized under prompt corrective action provisions (at least), ratio	0.050	0.050
Boston Private Bank | Basel III Minimum with Capital Conservation Buffer
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	0.085	0.085
Total risk-based capital
Total risk-based capital, for capital adequacy purposes (at least), ratio	0.105	0.105
Tier 1 leverage capital
Tier 1 leverage capital, for capital adequacy purposes (at least), ratio	0.040	0.040