UNAUDITED CONSOLIDATED STATEMENTS OF CHANGES IN SHAREHOLDERS’ EQUITY (Parenthetical) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended		12 Months Ended
	Jan. 01, 2023	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023	Dec. 31, 2022
Dividends declared per share (in dollars per share)		$ 0.11	$ 0.10	$ 0.33	$ 0.30
Restricted stock, shares cancelled upon vesting for employee withholdings (in shares)				139,585	74,625,000,000
Accounting Standards Update [Extensible List]						Accounting Standards Update 2022-02
Retained earnings		$ (2,048,042)		$ (2,048,042)
Deferred income tax expense (benefit)				$ 19,013	$ (96,622)
Restricted Stock
Restricted stock, shares cancelled upon forfeitures (in shares)				3,026	0
Restricted Stock | Cambridge Bancorp
Restricted stock, shares cancelled upon vesting for employee withholdings (in shares)		14,353		14,353
Restricted stock, shares cancelled upon forfeitures (in shares)		3,026		3,026
Issuance of restricted stock awards (in shares)		118,693		118,693
Restricted stock award shares, at fair value		$ 1,100		$ 1,100
Cumulative effect accounting adjustment
Retained earnings	$ 1,100
Deferred income tax expense (benefit)	$ 300