Employee Benefits - Additional Information (Details) - USD ($)			1 Months Ended				3 Months Ended		9 Months Ended		12 Months Ended	25 Months Ended	34 Months Ended
	Jul. 12, 2024	Nov. 29, 2021	May 31, 2024	Mar. 31, 2024	May 31, 2023	Mar. 31, 2023	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023	Dec. 31, 2023	Dec. 31, 2023	Sep. 30, 2024
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Service cost							$ 5,634,000	$ 6,339,000	$ 16,811,000	$ 19,016,000
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
Share-based payment arrangement, shares withheld for tax withholding obligation (in shares)									139,585	74,625,000,000
Cambridge Bancorp
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Pension benefit obligation	$ 35,300,000
Issuance of restricted stock awards (in shares)	38,900,000
Restricted Stock Units (RSUs)
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Grants in period (in shares)						318,577			562,454	318,577
Equity instruments other than options vested in period (in shares)									348,012	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)									88,948	28,690
Restricted Stock | Cambridge Bancorp
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Issuance of restricted stock awards (in shares)							118,693		118,693
Restricted stock award shares, at fair value							$ 1,100,000		$ 1,100,000
Share-based payment arrangement, shares withheld for tax withholding obligation (in shares)							14,353		14,353
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			301,441	108,984
Equity instruments other than options vested in period (in shares)									76,353	0
Defined Benefit Supplemental Executive Retirement Plan
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Service cost							$ 100,000	0	$ 100,000	$ 0
Outside Directors’ Retainer Continuance Plan
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Service cost							0	$ 0	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							$ 27,400,000		$ 27,400,000		$ 26,800,000	$ 26,800,000	$ 27,400,000
Period for recognition for unrecognized compensation expense related to unvested awards									1 year 8 months 12 days		2 years 2 months 12 days
Eastern Bankshares, Inc. 2021 Equity Incentive Plan | Cambridge Bancorp
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Retained employees pre-merger service credit	$ 3,000,000.0
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	3 years
Grants in period (in shares)				416,276			146,178
Number of shares available for grant (in shares)							3,969,089		3,969,089		4,872,494	4,872,494	3,969,089
Equity instruments other than options vested in period (in shares)									348,012,000,000	231,407,000,000
Shares withheld for tax withholding obligation (in shares)									$ 6,600,000	$ 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)									88,948,000,000	28,690,000,000
Shares withheld for tax withholding obligation (in shares)									$ 1,200,000	$ 500,000
Eastern Bankshares, Inc. 2021 Equity Incentive Plan | Restricted Stock | Cambridge Bancorp
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Issuance of restricted stock awards (in shares)	118,693
Restricted stock award shares, at fair value	$ 1,100,000
Post-combination fair value adjustment	$ 700,000
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			2 years 3 months 18 days
Grants in period (in shares)				234,091			67,350
Equity instruments other than options vested in period (in shares)									76,353	0
Shares withheld for tax withholding obligation (in shares)									$ 1,100,000