CONSOLIDATED STATEMENTS OF CHANGES IN SHAREHOLDERS’ EQUITY - USD ($) $ in Thousands		Total	Cumulative effect accounting adjustment		Common Stock	Additional Paid in Capital	Retained Earnings	Retained Earnings Cumulative effect accounting adjustment		Accumulated Other Comprehensive Income (Loss)	Unallocated Common Stock Held by ESOP
Beginning balance (in shares) at Dec. 31, 2021					186,305,332
Beginning balance at Dec. 31, 2021		$ 3,406,352	$ (20,098)	[1]	$ 1,863	$ 1,835,241	$ 1,768,653	$ (20,098)	[1]	$ (56,696)	$ (142,709)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Dividends to common shareholders	[2]	(66,539)					(66,539)
Repurchased common stock (in shares)					(10,112,272)
Repurchased common stock		(201,618)			$ (101)	(201,517)
Issuance of restricted stock awards (in shares)					31,559
Issuance of restricted stock awards		0			$ 1	(1)
Unvested restricted stock awards forfeited and subsequently cancelled (in shares)					(52,546)
Unvested restricted stock awards forfeited and subsequently cancelled		0			$ (1)	1
Share-based compensation		10,507				10,507
Net income		199,759					199,759
Other comprehensive income (loss), net of tax		(866,496)								(866,496)
ESOP shares committed to be released		9,923				4,910					5,013
Ending balance (in shares) at Dec. 31, 2022					176,172,073
Ending balance at Dec. 31, 2022		2,471,790	$ 822	[3]	$ 1,762	1,649,141	1,881,775	$ 822	[3]	(923,192)	(137,696)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Dividends to common shareholders	[2]	(67,020)					(67,020)
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)	[4]				207,100
Issuance of common stock under share-based compensation arrangements	[4]	(1,396)			$ 4	(1,400)
Share-based compensation		16,513				16,513
Net income		232,177					232,177
Other comprehensive income (loss), net of tax		314,840								314,840
ESOP shares committed to be released		$ 7,129				2,188					4,941
Ending balance (in shares) at Dec. 31, 2023		176,426,993			176,426,993
Ending 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]	(82,812)					(82,812)
Repurchased common stock (in shares)					(1,744,824)
Repurchased common stock		(27,683)			$ (17)	(27,666)
Issuance of restricted stock awards (in shares)					56,352
Issuance of restricted stock awards		0			$ 1	(1)
Restricted stock awards cancelled (in shares)	[5]				(18,513)
Restricted stock awards cancelled	[5]	(238)			$ (3)	(235)
Issuance of common stock under share-based compensation arrangements (in shares)	[4]				301,209
Issuance of common stock under share-based compensation arrangements	[4]	(2,010)			$ 4	(2,014)
Share-based compensation		19,269				19,269
Net income		119,561					119,561
Other comprehensive income (loss), net of tax		24,023								24,023
ESOP shares committed to be released		7,356				2,443					4,913
Common stock issued for acquisition (in shares)					38,769,562
Common stock issued for acquisition		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 upon merger close	[6]	1,053			$ 1	1,052
Pre-merger service credit for restricted share units	[7]	$ 3,030				3,030
Ending balance (in shares) at Dec. 31, 2024		213,909,472			213,909,472
Ending balance at Dec. 31, 2024		$ 3,611,967			$ 2,141	$ 2,237,494	$ 2,084,503			$ (584,329)	$ (127,842)

[1] Represents gross transition adjustment amount of $28.0 million, net of taxes of $7.9 million, to reflect the cumulative impact on retained earnings pursuant to the Company’s adoption of Accounting Standards Update 2016-13. Refer to Note 5, “Loans and Allowance for Credit Losses” for additional discussion.

[2] The Company declared cumulative cash dividends of $0.45, $0.41, and $0.40 per share of common stock during the years ended December 31, 2024, 2023, and 2022, respectively.

[3] 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” for additional discussion.

[4] Represents shares issued, net of employee tax withheld, upon the vesting of restricted stock units. Refer to Note 16, “Share-Based Compensation” for additional discussion.

[5] Includes 15,487 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 Company' 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 II, Item 8 in this Annual Report on Form 10-K 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” for additional discussion.

[7] 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” for additional discussion.