Share-Based Compensation - Schedule of Share-based Payment Arrangement, Restricted Stock, Restricted Stock Unit, and Performance Stock Unit, Activity (Details) - Eastern Bankshares, Inc. 2021 Equity Incentive Plan - $ / shares	1 Months Ended				3 Months Ended	12 Months Ended
	May 31, 2024	Mar. 31, 2024	May 31, 2023	Mar. 31, 2023	Sep. 30, 2024	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Number of Shares
Vested (in shares)								0
Restricted Stock Awards
Number of Shares
Non-vested stock at beginning of year (in shares)						420,400	525,460
Granted (in shares)	56,352		47,820			56,352	47,820	31,559
Vested (in shares)						(275,474)	(152,880)	(136,609)
Forfeited (in shares)						(3,026)	0
Converted in connection with merger (in shares)						118,693	0
Non-vested stock at end of year (in shares)						316,945	420,400	525,460
Weighted-Average Grant Price Per Share
Non-vested stock at beginning of year (in dollars per share)						$ 19.15	$ 20.08
Granted (in dollars per share)						13.84	11.50
Vested (in dollars per share)						16.27	19.95
Forfeited (in dollars per share)						14.87	0
Converted in connection with merger (in dollars per share)						14.87	0
Non-vested stock at end of year (in dollars per share)						$ 18.02	$ 19.15	$ 20.08
Restricted Stock Units
Number of Shares
Non-vested stock at beginning of year (in shares)						952,001	972,325
Granted (in shares)		416,276		318,577	146,178	562,454	318,577
Vested (in shares)						(372,179)	(302,908)	0
Forfeited (in shares)						(22,520)	(35,993)
Converted in connection with merger (in shares)						236,766	0
Non-vested stock at end of year (in shares)						1,356,522	952,001	972,325
Weighted-Average Grant Price Per Share
Non-vested stock at beginning of year (in dollars per share)						$ 19.46	$ 21.08
Granted (in dollars per share)						13.83	15.63
Vested (in dollars per share)						18.62	21.08
Forfeited (in dollars per share)						13.20	15.73
Converted in connection with merger (in dollars per share)						14.87	0
Non-vested stock at end of year (in dollars per share)						$ 16.55	$ 19.46	$ 21.08
Performance Stock Units
Number of Shares
Non-vested stock at beginning of year (in shares)						633,034	533,676
Granted (in shares)		234,091		108,984	67,350	301,441	108,984
Vested (in shares)						(76,353)	0	0
Forfeited (in shares)						0	(9,626)
Converted in connection with merger (in shares)						111,617	0
Non-vested stock at end of year (in shares)						969,739	633,034	533,676
Weighted-Average Grant Price Per Share
Non-vested stock at beginning of year (in dollars per share)						$ 19.40	$ 21.12
Granted (in dollars per share)						10.82	10.16
Vested (in dollars per share)						14.87	0
Forfeited (in dollars per share)						0	10.16
Converted in connection with merger (in dollars per share)						14.87	0
Non-vested stock at end of year (in dollars per share)						$ 16.63	$ 19.40	$ 21.12