Share-Based Compensation - Share-based Payment Arrangement, Restricted Stock, Restricted Stock Unit, and Performance Stock Unit, Activity (Details) - Eastern Bankshares, Inc. 2021 Equity Incentive Plan - USD ($) $ / shares in Units, $ in Millions	1 Months Ended					12 Months Ended			13 Months Ended	25 Months Ended
	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
Restricted Stock Awards
Number of Shares
Non-vested stock at beginning of year (in shares)						525,460	683,056
Granted (in shares)	47,820		31,559			47,820	31,559	683,056
Vested (in shares)						(152,880)	(136,609)
Forfeited (in shares)						0	(52,546)
Non-vested stock at end of year (in shares)					683,056	420,400	525,460	683,056	525,460	420,400
Weighted-Average Grant Price Per Share
Non-vested stock at beginning of year (in dollars per share)						$ 20.08	$ 20.13
Granted (in dollars per share)						11.50	19.17
Vested (in dollars per share)						19.95	20.13
Forfeited (in dollars per share)						0	20.08
Non-vested stock at end of year (in dollars per share)					$ 20.13	$ 19.15	$ 20.08	$ 20.13	$ 20.08	$ 19.15
Total grant date fair value								$ 13.7
Restricted Stock Units
Number of Shares
Non-vested stock at beginning of year (in shares)						972,325	0
Granted (in shares)		318,577		978,364		318,577	978,364	0
Vested (in shares)						(302,908)	0
Forfeited (in shares)						(35,993)	(6,039)
Non-vested stock at end of year (in shares)					0	952,001	972,325	0	972,325	952,001
Weighted-Average Grant Price Per Share
Non-vested stock at beginning of year (in dollars per share)						$ 21.08	$ 0
Granted (in dollars per share)						15.63	21.08
Vested (in dollars per share)						21.08	0
Forfeited (in dollars per share)						15.73	21.08
Non-vested stock at end of year (in dollars per share)					$ 0	$ 19.46	$ 21.08	$ 0	$ 21.08	$ 19.46
Share-based payment arrangement, shares withheld for tax withholding obligation (in shares)						95,808
Performance Stock Units
Number of Shares
Non-vested stock at beginning of year (in shares)						533,676	0
Granted (in shares)		108,984		533,676	0	108,984	533,676
Vested (in shares)									0	0
Forfeited (in shares)						(9,626)	0
Non-vested stock at end of year (in shares)					0	633,034	533,676	0	533,676	633,034
Weighted-Average Grant Price Per Share
Non-vested stock at beginning of year (in dollars per share)						$ 21.12	$ 0
Granted (in dollars per share)						10.16	21.12
Forfeited (in dollars per share)						10.16	0
Non-vested stock at end of year (in dollars per share)					$ 0	$ 19.40	$ 21.12	$ 0	$ 21.12	$ 19.40