Share-Based Compensation - Share-based Compensation Expense Under the 2021 Plan and the Related Tax Benefit (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation expense	$ 16,047	$ 10,507	$ 0
Eastern Bankshares, Inc. 2021 Equity Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation expense	16,500	10,500	200
Related tax benefit	$ 4,700	$ 3,000	$ 100