Regulatory Capital Requirements (Summary of Regulatory Capital Requirements) (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Bank actual capital amount
Common equity Tier 1 capital ratio, Actual, Amount	$ 780,962	$ 745,103
Tier 1 leverage capital	790,527	765,089
Risk-based capital:
Tier 1	790,527	765,089
Total risk-based capital	$ 927,515	$ 899,563
Bank actual capital ratio
Common equity Tier 1 capital ratio, Actual, Ratio (as a percent)	11.44%	11.94%
Tier 1 leverage capital ratio (as a percent)	10.28%	10.58%
Risk-based capital:
Tier 1 (as a percent)	11.58%	12.26%
Total (as a percent)	13.59%	14.42%
Minimum capital adequacy amount
Common equity Tier 1 capital ratio, Minimum Required for Capital Adequacy Purposes, Amount	$ 307,197	$ 280,818
Tier 1 leverage capital	307,598	289,259
Risk-based capital:
Tier 1	409,599	374,432
Total	$ 545,999	$ 499,064
Minimum capital adequacy ratio
Common equity Tier 1 capital ratio, Minimum Required for Capital Adequacy Purposes, Ratio (as a percent)	4.50%	4.50%
Tier 1 leverage capital ratio (as a percent)	4.00%	4.00%
Risk-based capital:
Tier 1 (as a percent)	6.00%	6.00%
Total (as a percent)	8.00%	8.00%
Minimum Required for Fully Phased in Capital Adequacy Purposes plus Capital Conservation Buffer
Common equity Tier 1 capital ratio	$ 477,861	$ 436,828
Tier 1 leverage capital ratio	307,598	289,259
Risk-based capital:
Tier 1 risk-based capital ratio	580,266	530,445
Total risk-based capital ratio	$ 716,623	$ 655,022
Minimum Required for Fully Phased in Capital Adequacy Purposes plus Capital Conservation Buffer
Common equity Tier 1 capital ratio (as a percent)	7.00%	7.00%
Tier 1 leverage capital ratio (as a percent)	4.00%	4.00%
Risk-based capital:
Tier 1 (as a percent)	8.50%	8.50%
Total (as a percent)	10.50%	10.50%
Brookline Bank
Bank actual capital amount
Common equity Tier 1 capital ratio, Actual, Amount	$ 513,311	$ 495,798
Tier 1 leverage capital	513,311	506,277
Risk-based capital:
Tier 1	513,311	506,277
Total risk-based capital	$ 555,474	$ 545,533
Bank actual capital ratio
Common equity Tier 1 capital ratio, Actual, Ratio (as a percent)	11.44%	12.06%
Tier 1 leverage capital ratio (as a percent)	10.42%	11.02%
Risk-based capital:
Tier 1 (as a percent)	11.44%	12.32%
Total (as a percent)	12.38%	13.27%
Minimum capital adequacy amount
Common equity Tier 1 capital ratio, Minimum Required for Capital Adequacy Purposes, Amount	$ 201,914	$ 184,999
Tier 1 leverage capital	197,048	183,767
Risk-based capital:
Tier 1	269,219	246,563
Total	$ 358,949	$ 328,882
Minimum capital adequacy ratio
Common equity Tier 1 capital ratio, Minimum Required for Capital Adequacy Purposes, Ratio (as a percent)	4.50%	4.50%
Tier 1 leverage capital ratio (as a percent)	4.00%	4.00%
Risk-based capital:
Tier 1 (as a percent)	6.00%	6.00%
Total (as a percent)	8.00%	8.00%
Minimum Required for Fully Phased in Capital Adequacy Purposes plus Capital Conservation Buffer
Common equity Tier 1 capital ratio	$ 314,089	$ 287,777
Tier 1 leverage capital ratio	197,048	183,767
Risk-based capital:
Tier 1 risk-based capital ratio	381,394	349,298
Total risk-based capital ratio	$ 471,121	$ 431,658
Minimum Required for Fully Phased in Capital Adequacy Purposes plus Capital Conservation Buffer
Common equity Tier 1 capital ratio (as a percent)	7.00%	7.00%
Tier 1 leverage capital ratio (as a percent)	4.00%	4.00%
Risk-based capital:
Tier 1 (as a percent)	8.50%	8.50%
Total (as a percent)	10.50%	10.50%
Classified as well capitalized
Common equity Tier 1 capital ratio, Minimum Required to Be Considered Well-Capitalized, Amount	$ 291,654	$ 267,221
Tier 1 leverage capital	246,310	229,708
Risk-based capital:
Tier 1	358,959	328,751
Total	$ 448,687	$ 411,102
Classified as well capitalized ratio
Common equity Tier 1 capital ratio, Minimum Required to Be Considered Well-Capitalized, Ratio (as a percent)	6.50%	6.50%
Tier 1 leverage capital ratio (as a percent)	5.00%	5.00%
Risk-based capital:
Tier 1 (as a percent)	8.00%	8.00%
Total (as a percent)	10.00%	10.00%
BankRI
Bank actual capital amount
Common equity Tier 1 capital ratio, Actual, Amount	$ 240,362	$ 209,670
Tier 1 leverage capital	240,362	209,670
Risk-based capital:
Tier 1	240,362	209,670
Total risk-based capital	$ 258,719	$ 227,674
Bank actual capital ratio
Common equity Tier 1 capital ratio, Actual, Ratio (as a percent)	11.75%	11.37%
Tier 1 leverage capital ratio (as a percent)	9.97%	9.35%
Risk-based capital:
Tier 1 (as a percent)	11.75%	11.37%
Total (as a percent)	12.65%	12.35%
Minimum capital adequacy amount
Common equity Tier 1 capital ratio, Minimum Required for Capital Adequacy Purposes, Amount	$ 92,054	$ 82,983
Tier 1 leverage capital	96,434	89,698
Risk-based capital:
Tier 1	122,738	110,644
Total	$ 163,617	$ 147,481
Minimum capital adequacy ratio
Common equity Tier 1 capital ratio, Minimum Required for Capital Adequacy Purposes, Ratio (as a percent)	4.50%	4.50%
Tier 1 leverage capital ratio (as a percent)	4.00%	4.00%
Risk-based capital:
Tier 1 (as a percent)	6.00%	6.00%
Total (as a percent)	8.00%	8.00%
Minimum Required for Fully Phased in Capital Adequacy Purposes plus Capital Conservation Buffer
Common equity Tier 1 capital ratio	$ 143,194	$ 129,084
Tier 1 leverage capital ratio	96,434	89,698
Risk-based capital:
Tier 1 risk-based capital ratio	173,879	156,745
Total risk-based capital ratio	$ 214,747	$ 193,569
Minimum Required for Fully Phased in Capital Adequacy Purposes plus Capital Conservation Buffer
Common equity Tier 1 capital ratio (as a percent)	7.00%	7.00%
Tier 1 leverage capital ratio (as a percent)	4.00%	4.00%
Risk-based capital:
Tier 1 (as a percent)	8.50%	8.50%
Total (as a percent)	10.50%	10.50%
Classified as well capitalized
Common equity Tier 1 capital ratio, Minimum Required to Be Considered Well-Capitalized, Amount	$ 132,966	$ 119,864
Tier 1 leverage capital	120,543	112,123
Risk-based capital:
Tier 1	163,651	147,525
Total	$ 204,521	$ 184,351
Classified as well capitalized ratio
Common equity Tier 1 capital ratio, Minimum Required to Be Considered Well-Capitalized, Ratio (as a percent)	6.50%	6.50%
Tier 1 leverage capital ratio (as a percent)	5.00%	5.00%
Risk-based capital:
Tier 1 (as a percent)	8.00%	8.00%
Total (as a percent)	10.00%	10.00%
First Ipswich
Bank actual capital amount
Common equity Tier 1 capital ratio, Actual, Amount	$ 41,320	$ 39,655
Tier 1 leverage capital	41,320	39,655
Risk-based capital:
Tier 1	41,320	39,655
Total risk-based capital	$ 43,762	$ 42,944
Bank actual capital ratio
Common equity Tier 1 capital ratio, Actual, Ratio (as a percent)	13.45%	13.91%
Tier 1 leverage capital ratio (as a percent)	8.80%	9.59%
Risk-based capital:
Tier 1 (as a percent)	13.45%	13.91%
Total (as a percent)	14.24%	15.06%
Minimum capital adequacy amount
Common equity Tier 1 capital ratio, Minimum Required for Capital Adequacy Purposes, Amount	$ 13,825	$ 12,829
Tier 1 leverage capital	18,782	16,540
Risk-based capital:
Tier 1	18,433	17,105
Total	$ 24,585	$ 22,812
Minimum capital adequacy ratio
Common equity Tier 1 capital ratio, Minimum Required for Capital Adequacy Purposes, Ratio (as a percent)	4.50%	4.50%
Tier 1 leverage capital ratio (as a percent)	4.00%	4.00%
Risk-based capital:
Tier 1 (as a percent)	6.00%	6.00%
Total (as a percent)	8.00%	8.00%
Minimum Required for Fully Phased in Capital Adequacy Purposes plus Capital Conservation Buffer
Common equity Tier 1 capital ratio	$ 21,505	$ 19,956
Tier 1 leverage capital ratio	18,782	16,540
Risk-based capital:
Tier 1 risk-based capital ratio	26,113	24,232
Total risk-based capital ratio	$ 32,268	$ 29,941
Minimum Required for Fully Phased in Capital Adequacy Purposes plus Capital Conservation Buffer
Common equity Tier 1 capital ratio (as a percent)	7.00%	7.00%
Tier 1 leverage capital ratio (as a percent)	4.00%	4.00%
Risk-based capital:
Tier 1 (as a percent)	8.50%	8.50%
Total (as a percent)	10.50%	10.50%
Classified as well capitalized
Common equity Tier 1 capital ratio, Minimum Required to Be Considered Well-Capitalized, Amount	$ 19,969	$ 18,530
Tier 1 leverage capital	23,477	20,675
Risk-based capital:
Tier 1	24,577	22,807
Total	$ 30,732	$ 28,515
Classified as well capitalized ratio
Common equity Tier 1 capital ratio, Minimum Required to Be Considered Well-Capitalized, Ratio (as a percent)	6.50%	6.50%
Tier 1 leverage capital ratio (as a percent)	5.00%	5.00%
Risk-based capital:
Tier 1 (as a percent)	8.00%	8.00%
Total (as a percent)	10.00%	10.00%