Employee Benefits - Additional Information (Details) - USD ($)		1 Months Ended				3 Months Ended		6 Months Ended		12 Months Ended	25 Months Ended	31 Months Ended
	Nov. 29, 2021	May 31, 2024	Mar. 31, 2024	May 31, 2023	Mar. 31, 2023	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023	Dec. 31, 2023	Dec. 31, 2023	Jun. 30, 2024
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Discretionary employer contributions to defined benefit plans						$ 0	$ 0	$ 0	$ 0
Reduction of shares available to be issued upon exercise of stock options with each additional restricted stock grant	3
Restricted Stock Units (RSUs)
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Grants in period (in shares)					318,577			416,276	318,577
Equity instruments other than options vested in period (in shares)								303,015	231,407
Restricted Stock
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Grants in period (in shares)								56,352	47,820
Equity instruments other than options vested in period (in shares)								47,820	28,690
Performance Shares
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Award vesting period					3 years
Grants in period (in shares)					108,984			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						$ 27,200,000		$ 27,200,000		$ 26,800,000	$ 26,800,000	$ 27,200,000
Period for recognition for unrecognized compensation expense related to unvested awards								1 year 10 months 24 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
Number of shares available for grant (in shares)						4,170,755		4,170,755		4,872,494	4,872,494	4,170,755
Equity instruments other than options vested in period (in shares)								303,015,000,000	231,407,000,000
Shares withheld for tax withholding obligation (in shares)								$ 5,900,000	$ 4,900,000
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	1 year		1 year
Grants in period (in shares)		56,352		47,820
Equity instruments other than options vested in period (in shares)								47,820,000,000	28,690,000,000
Shares withheld for tax withholding obligation (in shares)								$ 500,000	$ 500,000
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
Equity instruments other than options vested in period (in shares)								0	0