Business Combinations - Schedule Of Hypothetical Number Of Shares Required For Issuance For Equal Ownership (Details) - Holdco Merger - shares	Sep. 01, 2025	Aug. 31, 2025
Business Combination [Line Items]
Percentage Ownership	45.00%	100.00%
Company Stockholders
Business Combination [Line Items]
Percentage Ownership		55.18%
Legacy Brookline Stockholders
Business Combination [Line Items]
Percentage Ownership		44.82%
Brookline Bancorp, Inc.
Business Combination [Line Items]
Number of Legacy Brookline Outstanding Shares (in shares)		200,150,309
Brookline Bancorp, Inc. | Company Stockholders
Business Combination [Line Items]
Number of Legacy Brookline Outstanding Shares (in shares)	110,452,183	110,452,183
Brookline Bancorp, Inc. | Legacy Brookline Stockholders
Business Combination [Line Items]
Number of Legacy Brookline Outstanding Shares (in shares)		89,698,126