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- Definition

Share-based Compensation Arrangement By Share-based Payment Award, Reduction Of Shares Available To Be Issued Upon Exercise Of Stock Options With Each Additional Restricted Stock Grant

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- Definition

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.

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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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- Definition

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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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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- Definition

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

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The difference between the maximum number of shares (or other type of equity) authorized for issuance under the plan (including the effects of amendments and adjustments), and the sum of: 1) the number of shares (or other type of equity) already issued upon exercise of options or other equity-based awards under the plan; and 2) shares (or other type of equity) reserved for issuance on granting of outstanding awards, net of cancellations and forfeitures, if applicable.

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- Definition

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

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- Definition

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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Number of shares used to settle grantee's tax withholding obligation for award under share-based payment arrangement.

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Employee Benefits - Additional Information (Details) - USD ($)		1 Months Ended		3 Months Ended		12 Months Ended	49 Months Ended	52 Months Ended
	Nov. 29, 2021	Mar. 31, 2026	Mar. 31, 2025	Mar. 31, 2026	Mar. 31, 2025	Dec. 31, 2025	Dec. 31, 2025	Mar. 31, 2026
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Discretionary employer contributions to defined benefit plans				$ 0	$ 0
Reduction of shares available to be issued upon exercise of stock options with each additional restricted stock grant	3
Share-based payment arrangement, shares withheld for tax withholding obligation (in shares)				204,278	330,038
Eastern Bankshares, Inc. 2021 Equity Incentive Plan
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Number of shares authorized (in shares)	26,146,141
Options granted in period (in shares)							0	0
Unrecognized compensation expense related to unvested awards		$ 29,100,000		$ 29,100,000		$ 20,800,000	$ 20,800,000	$ 29,100,000
Period for recognition for unrecognized compensation expense related to unvested awards				1 year 10 months 24 days		1 year 7 months 6 days
Eastern Bankshares, Inc. 2021 Equity Incentive Plan \| Restricted Stock Units (RSUs)
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Number of shares authorized (in shares)	7,470,326
Award vesting period		3 years	3 years
Number of shares available for grant (in shares)		2,501,462		2,501,462		3,134,086	3,134,086	2,501,462
Eastern Bankshares, Inc. 2021 Equity Incentive Plan \| Share-based Payment Arrangement, Option
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Number of shares authorized (in shares)	18,675,815
Award expiration period	10 years
Number of shares available for grant (in shares)		18,675,815		18,675,815		18,675,815	18,675,815	18,675,815
Eastern Bankshares, Inc. 2021 Equity Incentive Plan \| Restricted Stock
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Award vesting period	5 years
Eastern Bankshares, Inc. 2021 Equity Incentive Plan \| Performance Shares
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Award vesting period		2 years 9 months 18 days	2 years 9 months 18 days