UNAUDITED CONSOLIDATED STATEMENTS OF CHANGES IN SHAREHOLDERS’ EQUITY - USD ($) $ in Thousands		Total	Cumulative effect accounting adjustment	[1]	Common Stock	Additional Paid in Capital	Retained Earnings	Retained Earnings Cumulative effect accounting adjustment	[1]	Accumulated Other Comprehensive Loss	Unallocated Common Stock Held by ESOP
Beginning balance (in shares) at Dec. 31, 2022					176,172,073
Beginning Balance at Dec. 31, 2022		$ 2,471,790	$ 822		$ 1,762	$ 1,649,141	$ 1,881,775	$ 822		$ (923,192)	$ (137,696)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Dividends to common shareholders	[2]	(49,046)					(49,046)
Issuance of restricted stock awards (in shares)					47,820
Issuance of restricted stock awards		0			$ 1	(1)
Issuance of common stock under share-based compensation arrangements (in shares)	[3]				156,782
Issuance of common stock under share-based compensation arrangements	[3]	(1,164)			$ 3	(1,167)
Share-based compensation		11,387				11,387
Net (loss) income		(86,326)					(86,326)
Other comprehensive income (loss), net of tax		93,610								93,610
ESOP shares committed to be released		5,480				1,776					3,704
Ending balance (in shares) at Sep. 30, 2023					176,376,675
Ending balance at Sep. 30, 2023		2,446,553			$ 1,766	1,661,136	1,747,225			(829,582)	(133,992)
Beginning balance (in shares) at Jun. 30, 2023					176,376,675
Beginning Balance at Jun. 30, 2023		2,526,772			$ 1,766	1,656,750	1,704,470			(700,982)	(135,232)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Dividends to common shareholders	[4]	(16,358)					(16,358)
Share-based compensation		3,675				3,675
Net (loss) income		59,113					59,113
Other comprehensive income (loss), net of tax		(128,600)								(128,600)
ESOP shares committed to be released		1,951				711					1,240
Ending balance (in shares) at Sep. 30, 2023					176,376,675
Ending balance at Sep. 30, 2023		$ 2,446,553			$ 1,766	1,661,136	1,747,225			(829,582)	(133,992)
Beginning balance (in shares) at Dec. 31, 2023		176,426,993			176,426,993
Beginning Balance at Dec. 31, 2023		$ 2,974,855			$ 1,767	1,666,441	2,047,754			(608,352)	(132,755)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Dividends to common shareholders	[2]	(58,502)					(58,502)
Repurchased common stock (in shares)					(836,399)
Repurchased common stock		(12,628)			$ (8)	(12,620)
Issuance of restricted stock awards (in shares)					56,352
Issuance of restricted stock awards		0			$ 1	(1)
Restricted share awards cancelled (in shares)	[5]				(17,379)
Restricted share awards cancelled	[5]	(216)			$ (2)	(214)
Issuance of common stock under share-based compensation arrangements (in shares)	[3]				284,780
Issuance of common stock under share-based compensation arrangements	[3]	(1,880)			$ 3	(1,883)
Share-based compensation		13,576				13,576
Net (loss) income		58,790					58,790
Other comprehensive income (loss), net of tax		112,241								112,241
ESOP shares committed to be released		5,256				1,578					3,678
Common stock issued for merger (in shares)					38,769,562
Common stock issued for merger		576,504			$ 388	576,116
Stock issuance costs		(941)				(941)
Cambridge Bancorp restricted share awards converted to restricted share awards of the Company at fair value (in shares)	[6]				118,693
Cambridge Bancorp restricted share awards converted to restricted share awards of the Company at fair value	[6]	1,053			$ 1	1,052
Pre-merger service credit for restricted share units	[7]	$ 3,030				3,030
Ending balance (in shares) at Sep. 30, 2024		214,802,602			214,802,602
Ending balance at Sep. 30, 2024		$ 3,671,138			$ 2,150	2,246,134	2,048,042			(496,111)	(129,077)
Beginning balance (in shares) at Jun. 30, 2024					176,687,829
Beginning Balance at Jun. 30, 2024		2,967,473			$ 1,770	1,673,722	2,076,566			(654,290)	(130,295)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Dividends to common shareholders	[4]	(22,336)					(22,336)
Repurchased common stock (in shares)					(836,399)
Repurchased common stock		(12,628)			$ (8)	(12,620)
Restricted share awards cancelled (in shares)	[8]				(17,379)
Restricted share awards cancelled	[8]	(216)			$ (2)	(214)
Issuance of common stock under share-based compensation arrangements (in shares)	[9]				80,296
Issuance of common stock under share-based compensation arrangements	[9]	(618)			$ 1	(619)
Share-based compensation		5,745				5,745
Net (loss) income		(6,188)					(6,188)
Other comprehensive income (loss), net of tax		158,179								158,179
ESOP shares committed to be released		2,081				863					1,218
Common stock issued for merger (in shares)					38,769,562
Common stock issued for merger		576,504			$ 388	576,116
Stock issuance costs		(941)				(941)
Cambridge Bancorp restricted share awards converted to restricted share awards of the Company at fair value (in shares)	[10]				118,693
Cambridge Bancorp restricted share awards converted to restricted share awards of the Company at fair value	[10]	1,053			$ 1	1,052
Pre-merger service credit for restricted share units	[11]	$ 3,030				3,030
Ending balance (in shares) at Sep. 30, 2024		214,802,602			214,802,602
Ending balance at Sep. 30, 2024		$ 3,671,138			$ 2,150	$ 2,246,134	$ 2,048,042			$ (496,111)	$ (129,077)

[1] Represents gross transition adjustment amount of $1.1 million, net of taxes of $0.3 million, to reflect the cumulative impact on retained earnings pursuant to the Company’s adoption of Accounting Standards Update 2022-02. Refer to Note 5, “Loans and Allowance for Credit Losses” within the Notes to the Unaudited Consolidated Financial Statements included in Part I, Item 1 in this Quarterly Report on Form 10-Q for additional discussion.

[2] The Company declared cumulative cash dividends of $0.33 and $0.30 per share of common stock during the nine months ended September 30, 2024 and 2023, respectively.

[3] Represents shares issued, net of employee tax withheld, upon the vesting of restricted stock units. Refer to Note 11, “Employee Benefits” within the Notes to the Unaudited Consolidated Financial Statements included in Part I, Item 1 in this Quarterly Report on Form 10-Q for additional discussion.

[4] The Company declared quarterly cash dividends of $0.11 and $0.10 per share of common stock during the three months ended September 30, 2024 and 2023, respectively.

[5] Includes 14,353 restricted stock award shares cancelled upon vesting for employee payroll tax withholding and 3,026 restricted stock award shares cancelled upon forfeiture. Shares withheld relate to awards issued by Cambridge Bancorp to its employees which were converted to awards of the Company upon completion of the merger. Refer to Note 3, “Mergers and Acquisitions” within the Notes to the Unaudited Consolidated Financial Statements included in Part I, Item 1 in this Quarterly Report on Form 10-Q for additional discussion.

[6] The Company issued 118,693 restricted stock award shares with a fair value of $1.1 million as part of the purchase consideration for the merger. Refer to Note 3, “Mergers and Acquisitions” within the Notes to the Unaudited Consolidated Financial Statements included in Part I, Item 1 in this Quarterly Report on Form 10-Q for additional discussion.

[7] Represents credit for service for Company employees retained following the merger related to restricted share unit awards converted to Company restricted share units at the merger date of July 12, 2024. Refer to Note 3, “Mergers and Acquisitions” within the Notes to the Unaudited Consolidated Financial Statements included in Part I, Item 1 in this Quarterly Report on Form 10-Q for additional discussion.

[8] Includes 14,353 restricted stock award shares cancelled upon vesting for employee payroll tax withholding and 3,026 restricted stock award shares cancelled upon forfeiture. Shares withheld relate to awards issued by Cambridge Bancorp to its employees which were converted to awards of the Company upon completion of our merger with Cambridge Bancorp effective July 12, 2024 (“the merger”). Refer to Note 3, “Mergers and Acquisitions” within the Notes to the Unaudited Consolidated Financial Statements included in Part I, Item 1 in this Quarterly Report on Form 10-Q for additional discussion.

[9] Represents shares issued, net of employee tax withheld, upon the vesting of restricted stock units. Refer to Note 11, “Employee Benefits” within the Notes to the Unaudited Consolidated Financial Statements included in Part I, Item 1 in this Quarterly Report on Form 10-Q for additional discussion.

[10] The Company issued 118,693 restricted stock award shares with a fair value of $1.1 million as part of the purchase consideration for the merger. Refer to Note 3, “Mergers and Acquisitions” within the Notes to the Unaudited Consolidated Financial Statements included in Part I, Item 1 in this Quarterly Report on Form 10-Q for additional discussion.

[11] Represents credit for service for former employees of Cambridge Bancorp retained following the merger related to restricted share unit awards converted to Company restricted share units at the merger date of July 12, 2024. Refer to Note 3, “Mergers and Acquisitions” within the Notes to the Unaudited Consolidated Financial Statements included in Part I, Item 1 in this Quarterly Report on Form 10-Q for additional discussion.