Share-Based Compensation - Narrative (Details) - USD ($) $ in Millions			1 Months Ended					12 Months Ended			13 Months Ended	25 Months Ended
	Nov. 29, 2021	Oct. 14, 2020	May 31, 2023	Mar. 31, 2023	May 31, 2022	Mar. 31, 2022	Dec. 31, 2021	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2022	Dec. 31, 2023
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Amount borrowed under ESOP		$ 149.4
Total shares (in shares)		14,940,652						14,916,133	14,920,942		14,920,942	14,916,133
ESOP loan term		30 years
Shares committed to be released (in shares)								103,230	104,464		104,464	103,230
Annual amount from 2022 through 2049
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares committed to be released (in shares)								495,339				495,339
Annual amount for 2050
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares committed to be released (in shares)								392,141				392,141
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
Unrecognized compensation expense related to unvested awards								$ 26.8	$ 34.6		$ 34.6	$ 26.8
Period for recognition for unrecognized compensation expense related to unvested awards								2 years 2 months 12 days	3 years 3 months 18 days
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
Grants in period (in shares)				318,577		978,364		318,577	978,364	0
Number of shares available for grant (in shares)								4,872,494	5,302,256		5,302,256	4,872,494
Equity instruments other than options vested in period (in shares)								302,908	0
Shares withheld for tax withholding obligation (in shares)								$ 6.4
Eastern Bankshares, Inc. 2021 Equity Incentive Plan | Restricted Stock Units | Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period						3 years
Eastern Bankshares, Inc. 2021 Equity Incentive Plan | Restricted Stock Units | Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period						5 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
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)			47,820		31,559			47,820	31,559	683,056
Equity instruments other than options vested in period (in shares)								152,880	136,609
Shares withheld for tax withholding obligation (in shares)								$ 3.0	$ 2.7
Eastern Bankshares, Inc. 2021 Equity Incentive Plan | Performance Stock Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period				3 years		3 years
Grants in period (in shares)				108,984		533,676	0	108,984	533,676
Equity instruments other than options vested in period (in shares)											0	0