Employee Benefits - Narrative (Details) - USD ($)		1 Months Ended		3 Months Ended		12 Months Ended	25 Months Ended	28 Months Ended
	Nov. 29, 2021	Mar. 31, 2024	Mar. 31, 2023	Mar. 31, 2024	Mar. 31, 2023	Dec. 31, 2023	Dec. 31, 2023	Mar. 31, 2024
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 (in shares)	3
Restricted Stock Units (RSUs)
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Grants in period (in shares)				416,276	318,577
Equity instruments other than options vested in period (in shares)				303,015	230,768
Restricted Stock
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Grants in period (in shares)				0	0
Equity instruments other than options vested in period (in shares)				0	0
Performance Shares
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Grants in period (in shares)				234,091	108,984
Equity instruments other than options vested in period (in shares)				0	0
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		$ 31,000,000		$ 31,000,000		$ 26,800,000	$ 26,800,000	$ 31,000,000
Period for recognition for unrecognized compensation expense related to unvested awards				2 years 2 months 12 days		2 years 2 months 12 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
Grants in period (in shares)		416,276	318,577
Number of shares available for grant (in shares)		4,227,107		4,227,107		4,872,494	4,872,494	4,227,107
Equity instruments other than options vested in period (in shares)				303,015,000,000	230,768,000,000
Shares withheld for tax withholding obligation				$ 5,900,000	$ 4,900,000
Eastern Bankshares, Inc. 2021 Equity Incentive Plan | Restricted Stock Units (RSUs) | Minimum
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Award vesting period			3 years
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
Equity instruments other than options vested in period (in shares)				0	0
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
Grants in period (in shares)		234,091	108,984
Equity instruments other than options vested in period (in shares)				0	0