Fair Value of Financial Instruments - Information About Significant Unobservable Inputs (Details) $ in Thousands	Mar. 31, 2025 USD ($)	Dec. 31, 2024 USD ($)
Quantitative information for Level 3 Fair Value Measurements Assets
Estimated Fair Value	$ 882,353	$ 895,034
Municipal obligations
Quantitative information for Level 3 Fair Value Measurements Assets
Estimated Fair Value	19,031	20,254
Corporate debt obligations
Quantitative information for Level 3 Fair Value Measurements Assets
Estimated Fair Value	12,404	$ 12,287
Level 3 \| Discounted Cash Flow \| Municipal obligations
Quantitative information for Level 3 Fair Value Measurements Assets
Estimated Fair Value	15,888
Level 3 \| Observable Bids \| Corporate debt obligations
Quantitative information for Level 3 Fair Value Measurements Assets
Estimated Fair Value	$ 2,441
Level 3 \| Minimum \| Discounted Cash Flow \| Measurement Input, Discount Rate \| Municipal obligations
Quantitative information for Level 3 Fair Value Measurements Assets
Debt securities, measurement input	0.000
Level 3 \| Maximum \| Discounted Cash Flow \| Measurement Input, Discount Rate \| Municipal obligations
Quantitative information for Level 3 Fair Value Measurements Assets
Debt securities, measurement input	0.0385
Level 3 \| Weighted Average \| Discounted Cash Flow \| Measurement Input, Discount Rate \| Municipal obligations
Quantitative information for Level 3 Fair Value Measurements Assets
Debt securities, measurement input	0.0188

BROOKLINE BANCORP INC 10-Q