Basis of Presentation	6 Months Ended
Jun. 30, 2025
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Basis of Presentation	Basis of Presentation Overview The Company is a bank holding company (within the meaning of the Bank Holding Company Act of 1956, as amended) and the parent of Brookline Bank, a Massachusetts-chartered trust company; BankRI, a Rhode Island-chartered financial institution; and PCSB Bank, a New York-chartered commercial bank. The Banks are members of the Federal Reserve System. The Company is also the parent of Clarendon Private. The Company's primary business is to provide commercial, business and retail banking services to its corporate, municipal and retail customers through the Banks and its non-bank subsidiaries. Brookline Bank, which includes its wholly-owned subsidiaries, Longwood Securities Corp., Eastern Funding and First Ipswich Insurance Agency, operates 28 full-service banking offices in the Greater Boston metropolitan area with three additional lending offices. BankRI, which includes its wholly-owned subsidiaries, Acorn Insurance Agency, BRI Realty Corp., BRI Investment Corp. and its wholly-owned subsidiary, BRI MSC Corp., operates 22 full-service banking offices in the greater Providence, Rhode Island area. PCSB Bank, which includes its wholly-owned subsidiary, UpCounty Realty Corp., operates 14 full-service banking offices in the Lower Hudson Valley of New York. Clarendon Private is a registered investment advisor with the SEC. Through Clarendon Private, the Company offers a wide range of wealth management services to individuals, families, endowments and foundations to help these clients meet their long-term financial goals. The Banks' activities include acceptance of commercial, municipal and retail deposits, origination of mortgage loans on commercial and residential real estate located principally in Central New England and the Lower Hudson Valley of New York State, origination of commercial loans and leases to small- and mid-sized businesses, investment in debt and equity securities, and the offering of cash management and wealth and investment advisory services. The Company also provides specialty equipment financing through its subsidiary Eastern Funding, which is based in New York City, New York, and Plainview, New York. The Company and the Banks are supervised, examined and regulated by the FRB. As a Massachusetts-chartered trust company, Brookline Bank is subject to supervision, examination and regulation by the Massachusetts Division of Banks. As a Rhode Island-chartered financial institution, BankRI is subject to supervision, examination and regulation by the Banking Division of the Rhode Island Department of Business Regulation. As a New York chartered commercial bank, PCSB Bank is subject to supervision, examination and regulation by the New York State Department of Financial Services. Clarendon Private is also subject to regulation by the SEC. The FDIC offers insurance coverage on all deposits up to $250,000 per depositor at each of the Banks. As FDIC-insured depository institutions, the Banks are also subject to supervision, examination and regulation by the FDIC. Basis of Financial Statement Presentation The unaudited consolidated financial statements of the Company presented herein have been prepared pursuant to the rules of the SEC for quarterly reports on Form 10-Q and do not include all of the information and note disclosures required by GAAP. In the opinion of management, all adjustments (consisting of normal recurring adjustments) and disclosures considered necessary for the fair presentation of the accompanying consolidated financial statements have been included. Interim results are not necessarily reflective of the results of the entire year. The accompanying unaudited consolidated financial statements should be read in conjunction with the audited consolidated financial statements and notes thereto included in the Annual Report on Form 10-K for the fiscal year ended December 31, 2024. The unaudited consolidated financial statements include the accounts of the Company and its wholly-owned subsidiaries. All significant intercompany transactions and balances are eliminated in consolidation. In preparing these consolidated financial statements, management is required to make significant estimates and assumptions that affect the reported amounts of assets, liabilities, income, expenses and disclosure of contingent assets and liabilities. Actual results could differ from those estimates based upon changing conditions, including economic conditions and future events. Material estimates that are particularly susceptible to significant changes in the near-term include the determination of the ACL and the determination of fair market values of assets and liabilities. The judgments used by management in applying these critical accounting policies may be affected by a further and prolonged deterioration in the economic environment, which may result in changes to future financial results. For example, subsequent evaluations of the loan and lease portfolio, in light of the factors then prevailing, may result in significant changes in the allowance for loan and lease losses in future periods, and the inability to collect outstanding principal may result in increased loan and lease losses. Reclassification Certain previously reported amounts have been reclassified to conform to the current year's presentation. Segment Reporting An operating segment is defined as a component of a business for which separate financial information is available that is evaluated regularly by the CODM in deciding how to allocate resources and evaluate performance. The Company is a bank holding company operating through a single business segment, which derives interest income on loan and lease products the Company offers to customers. Substantially all of the Company’s total revenues, pre-tax income, and assets is driven by the banking business. While revenue generating activities are aligned through our bank subsidiaries, expense activities, including funding costs, credit losses and operating expenses, are managed for the Company as a whole. As a result, detailed profitability for each subsidiary bank is not used by the CODM. The Chairman and Chief Executive Officer of the Company acts as the Company’s CODM. The CODM regularly reviews comprehensive financial information with the reported measures focused on net interest income and net income. This financial information reviewed is consistent with the information presented within the Company’s financial statements. The CODM uses the reported measures of net interest income and net income to assess performance by comparing to and monitoring against budget and prior year results. This information is used to manage resources to drive business and net earnings growth, including investment in key strategic priorities, as well as determine the Company's ability to return capital to shareholders.

The Company is a bank holding company (within the meaning of the Bank Holding Company Act of 1956, as amended) and the parent of Brookline Bank, a Massachusetts-chartered trust company; BankRI, a Rhode Island-chartered financial institution; and PCSB Bank, a New York-chartered commercial bank. The Banks are members of the Federal Reserve System. The Company is also the parent of Clarendon Private. The Company's primary business is to provide commercial, business and retail banking services to its corporate, municipal and retail customers through the Banks and its non-bank subsidiaries.

Brookline Bank, which includes its wholly-owned subsidiaries, Longwood Securities Corp., Eastern Funding and First Ipswich Insurance Agency, operates 28 full-service banking offices in the Greater Boston metropolitan area with three additional lending offices. BankRI, which includes its wholly-owned subsidiaries, Acorn Insurance Agency, BRI Realty Corp., BRI Investment Corp. and its wholly-owned subsidiary, BRI MSC Corp., operates 22 full-service banking offices in the greater Providence, Rhode Island area. PCSB Bank, which includes its wholly-owned subsidiary, UpCounty Realty Corp., operates 14 full-service banking offices in the Lower Hudson Valley of New York. Clarendon Private is a registered investment advisor with the SEC. Through Clarendon Private, the Company offers a wide range of wealth management services to individuals, families, endowments and foundations to help these clients meet their long-term financial goals.

The Banks' activities include acceptance of commercial, municipal and retail deposits, origination of mortgage loans on commercial and residential real estate located principally in Central New England and the Lower Hudson Valley of New York State, origination of commercial loans and leases to small- and mid-sized businesses, investment in debt and equity securities, and the offering of cash management and wealth and investment advisory services. The Company also provides specialty equipment financing through its subsidiary Eastern Funding, which is based in New York City, New York, and Plainview, New York.

The Company and the Banks are supervised, examined and regulated by the FRB. As a Massachusetts-chartered trust company, Brookline Bank is subject to supervision, examination and regulation by the Massachusetts Division of Banks. As a Rhode Island-chartered financial institution, BankRI is subject to supervision, examination and regulation by the Banking Division of the Rhode Island Department of Business Regulation. As a New York chartered commercial bank, PCSB Bank is subject to supervision, examination and regulation by the New York State Department of Financial Services. Clarendon Private is also subject to regulation by the SEC.

The FDIC offers insurance coverage on all deposits up to $250,000 per depositor at each of the Banks. As FDIC-insured depository institutions, the Banks are also subject to supervision, examination and regulation by the FDIC.

The unaudited consolidated financial statements of the Company presented herein have been prepared pursuant to the rules of the SEC for quarterly reports on Form 10-Q and do not include all of the information and note disclosures required by GAAP. In the opinion of management, all adjustments (consisting of normal recurring adjustments) and disclosures considered necessary for the fair presentation of the accompanying consolidated financial statements have been included. Interim results are not necessarily reflective of the results of the entire year. The accompanying unaudited consolidated financial statements should be read in conjunction with the audited consolidated financial statements and notes thereto included in the Annual Report on Form 10-K for the fiscal year ended December 31, 2024.

The unaudited consolidated financial statements include the accounts of the Company and its wholly-owned subsidiaries. All significant intercompany transactions and balances are eliminated in consolidation.

In preparing these consolidated financial statements, management is required to make significant estimates and assumptions that affect the reported amounts of assets, liabilities, income, expenses and disclosure of contingent assets and liabilities. Actual results could differ from those estimates based upon changing conditions, including economic conditions and future events. Material estimates that are particularly susceptible to significant changes in the near-term include the determination of the ACL and the determination of fair market values of assets and liabilities.

The judgments used by management in applying these critical accounting policies may be affected by a further and prolonged deterioration in the economic environment, which may result in changes to future financial results. For example, subsequent evaluations of the loan and lease portfolio, in light of the factors then prevailing, may result in significant changes in

An operating segment is defined as a component of a business for which separate financial information is available that is evaluated regularly by the CODM in deciding how to allocate resources and evaluate performance.

The Company is a bank holding company operating through a single business segment, which derives interest income on loan and lease products the Company offers to customers. Substantially all of the Company’s total revenues, pre-tax income, and assets is driven by the banking business. While revenue generating activities are aligned through our bank subsidiaries, expense activities, including funding costs, credit losses and operating expenses, are managed for the Company as a whole. As a result, detailed profitability for each subsidiary bank is not used by the CODM.

The Chairman and Chief Executive Officer of the Company acts as the Company’s CODM. The CODM regularly reviews comprehensive financial information with the reported measures focused on net interest income and net income. This financial information reviewed is consistent with the information presented within the Company’s financial statements.