Share-Based Compensation - Narrative (Details) - USD ($) $ in Millions				1 Months Ended				3 Months Ended	12 Months Ended			25 Months Ended	37 Months Ended
	Jul. 12, 2024	Nov. 29, 2021	Oct. 14, 2020	May 31, 2024	Mar. 31, 2024	May 31, 2023	Mar. 31, 2023	Sep. 30, 2024	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2023	Dec. 31, 2024
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Amount borrowed under ESOP			$ 149.4
Total shares (in shares)			14,940,652						14,776,579	14,916,133		14,916,133	14,776,579
ESOP loan term			30 years
Shares committed to be released (in shares)									103,078	103,230		103,230	103,078
Cambridge Bancorp
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Issuance of restricted stock awards (in shares)	38,900,000
Annual amount from 2022 through 2049
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares committed to be released (in shares)									495,672				495,672
Annual amount for 2050
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares committed to be released (in shares)									392,404				392,404
Restricted Stock Awards | 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									$ 1.1
Share-based payment arrangement, shares withheld for tax withholding obligation (in shares)									15,487
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
Reduction of shares available to be issued upon exercise of stock options with each additional restricted stock grant		3
Options granted in period (in shares)												0	0
Equity instruments other than options vested in period (in shares)											0
Share-based payment arrangement, shares withheld for tax withholding obligation (in shares)									147,323	95,808
Unrecognized compensation expense related to unvested awards									$ 21.4	$ 26.8		$ 26.8	$ 21.4
Period for recognition for unrecognized compensation expense related to unvested awards									1 year 4 months 24 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	$ 3.0
Eastern Bankshares, Inc. 2021 Equity Incentive Plan | Restricted Stock Units
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		318,577	146,178	562,454	318,577
Number of shares available for grant (in shares)									3,844,157	4,872,494		4,872,494	3,844,157
Equity instruments other than options vested in period (in shares)									372,179	302,908	0
Shares withheld for tax withholding obligation (in shares)									$ 6.9	$ 6.4
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
Eastern Bankshares, Inc. 2021 Equity Incentive Plan | Restricted Stock Awards
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			56,352	47,820	31,559
Equity instruments other than options vested in period (in shares)									275,474	152,880	136,609
Shares withheld for tax withholding obligation (in shares)									$ 4.5	$ 3.0	$ 2.7
Total grant date fair value									$ 0.8	$ 0.5	$ 0.6
Eastern Bankshares, Inc. 2021 Equity Incentive Plan | Restricted Stock Awards | 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	$ 1.1
Post-combination fair value adjustment	$ 0.7
Eastern Bankshares, Inc. 2021 Equity Incentive Plan | Performance Stock Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period					2 years 9 months 18 days		3 years	2 years 3 months 18 days
Grants in period (in shares)					234,091		108,984	67,350	301,441	108,984
Equity instruments other than options vested in period (in shares)									76,353	0	0
Shares withheld for tax withholding obligation (in shares)									$ 1.1