Share-Based Compensation - Additional Information (Details) - USD ($) $ in Millions		1 Months Ended				9 Months Ended		12 Months Ended	13 Months Ended	22 Months Ended
	Nov. 29, 2021	May 31, 2023	Mar. 31, 2023	May 31, 2022	Mar. 31, 2022	Sep. 30, 2023	Sep. 30, 2022	Dec. 31, 2022	Dec. 31, 2022	Sep. 30, 2023
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
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)					978,364	318,577	978,364
Equity instruments other than options vested in period (in shares)						231,407	0
Restricted Stock
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Grants in period (in shares)						47,820	31,559
Equity instruments other than options vested in period (in shares)						28,690	0
Performance Shares
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Award vesting period					3 years
Grants in period (in shares)					533,676	108,984	533,676
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
Equity instruments other than options vested in period (in shares)							0
Unrecognized compensation expense related to unvested awards						$ 32.6		$ 34.6	$ 34.6	$ 32.6
Period for recognition for unrecognized compensation expense related to unvested awards						2 years 6 months		3 years 3 months 18 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
Grants in period (in shares)			318,577
Number of shares available for grant (in shares)						4,830,074		5,302,256	5,302,256	4,830,074
Equity instruments other than options vested in period (in shares)						231,407,000,000			0
Shares withheld for tax withholding obligation (in shares)						$ 4.9
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 | Restricted Stock Units (RSUs) | 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
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
Equity instruments other than options vested in period (in shares)						28,690,000,000
Shares withheld for tax withholding obligation (in shares)						$ 0.5
Eastern Bankshares, Inc. 2021 Equity Incentive Plan | 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
Equity instruments other than options vested in period (in shares)									0	0