STOCK-BASED COMPENSATION - Equity incentive plan (Details) - USD ($) $ in Millions		9 Months Ended
Aug. 25, 2022	Sep. 30, 2022
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Granted (in shares)		299,481
Granted (in shares)		748,713
Expected future expense of non-vested options		$ 2.8
Restricted Stock
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Expected period of recognition (in years)		4 years 9 months 18 days
Expected future expense of non-vested RSUs		$ 3.4
Option
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Expected period of recognition (in years)		4 years 9 months 18 days
Blue Foundry Bancorp 2022 Equity Incentive Plan (Equity Plan)
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Number of shares authorized for grant (in shares)	3,993,150
Blue Foundry Bancorp 2022 Equity Incentive Plan (Equity Plan) \| Restricted Stock
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Number of shares authorized for grant (in shares)	1,140,900
Initial period before vesting begins (in years)	1 year
Granted (in shares)		299,481
Blue Foundry Bancorp 2022 Equity Incentive Plan (Equity Plan) \| Restricted Stock \| Minimum
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Vesting service period (in years)	5 years
Blue Foundry Bancorp 2022 Equity Incentive Plan (Equity Plan) \| Restricted Stock \| Maximum
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Vesting service period (in years)	7 years
Blue Foundry Bancorp 2022 Equity Incentive Plan (Equity Plan) \| Option
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Number of shares authorized for grant (in shares)	2,852,250
Initial period before vesting begins (in years)	1 year
Expiration period of options (in years)		10 years
Granted (in shares)		748,713
Expected life of stock options (in years)		6 years 6 months
Risk free interest rate		3.15%
Volatility rate		29.74%
Expected dividend yeild		0.88%
Blue Foundry Bancorp 2022 Equity Incentive Plan (Equity Plan) \| Option \| Minimum
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Vesting service period (in years)	5 years
Blue Foundry Bancorp 2022 Equity Incentive Plan (Equity Plan) \| Option \| Maximum
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Vesting service period (in years)	7 years

Share-Based Compensation Arrangement By Share-Based Payment Award, Vesting, Period After Grant

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Weighted-average period over which cost not yet recognized is expected to be recognized for award under share-based payment arrangement, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

+ References

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Amount of cost to be recognized for nonvested award under share-based payment arrangement. Excludes share and unit options.

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Amount of cost to be recognized for option under share-based payment arrangement.

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Period over which grantee's right to exercise award under share-based payment arrangement is no longer contingent on satisfaction of service or performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days. Includes, but is not limited to, combination of market, performance or service condition.

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The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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The estimated dividend rate (a percentage of the share price) to be paid (expected dividends) to holders of the underlying shares over the option's term.

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The estimated measure of the percentage by which a share price is expected to fluctuate during a period. Volatility also may be defined as a probability-weighted measure of the dispersion of returns about the mean. The volatility of a share price is the standard deviation of the continuously compounded rates of return on the share over a specified period. That is the same as the standard deviation of the differences in the natural logarithms of the stock prices plus dividends, if any, over the period.

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The risk-free interest rate assumption that is used in valuing an option on its own shares.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Number of shares authorized for issuance under share-based payment arrangement.

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Gross number of share options (or share units) granted during the period.

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Period from grant date that an equity-based award expires, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Expected term of award under share-based payment arrangement, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

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