Loans and Allowance for Credit Losses - Narrative (Details)			12 Months Ended
		Jul. 12, 2024 USD ($)	Dec. 31, 2024 USD ($) loan	Dec. 31, 2023 USD ($) loan	Dec. 31, 2022 USD ($) loan	Sep. 30, 2024 USD ($)	Dec. 31, 2021 USD ($)
Financing Receivable, Allowance for Credit Loss [Line Items]
Loan syndications, amount			$ 100,000,000
Financing receivable, excluding accrued interest, after allowance for credit loss			17,549,402,000	$ 13,799,367,000
FHLB advances			17,589,000	17,738,000
Debt, long-term and short-term, combined amount			93,900,000	48,216,000
Mortgage loans partially or wholly-owned by others and serviced by the Company			228,400,000	77,200,000
Proceeds from sales of commercial loans held for investment			0	189,296,000	$ 0
Realized loss on sales of commercial loans	[1]		0	2,738,000	0
Provision for allowance for loan losses			67,380,000	20,052,000	17,925,000
Provision for allowance for loan losses,excluding loans acquired through business combination			26,500,000
Financing receivable, excluding accrued interest, allowance for credit loss, writeoff			47,757,000	15,092,000	4,831,000
Initial reserve on PCD loans at merger			55,830,000
Financing receivable, allowance for credit loss, excluding accrued interest			$ 228,952,000	148,993,000	$ 142,211,000		$ 97,787,000
Maximum number of days required for special mention			90 days
Loans modified during the prior 12 months which had subsequently defaulted			$ 500,000	0
Financing Receivable, Modified, Commitment to Lend			300,000	0
Number of loans modified during the preceding 12 months that subsequently defaulted | loan					1
Amount of loans modified during the preceding 12 months that subsequently defaulted					$ 1,000,000.0
Amounts charged-off on TDRs					0
Expected Credit Losses
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing receivable, allowance for credit loss, excluding accrued interest						$ 5,600,000
Cambridge Bancorp
Financing Receivable, Allowance for Credit Loss [Line Items]
Provision for allowance for loan losses			40,900,000
Financing receivable, excluding accrued interest, charge-offs on PCD loans			19,800,000
Initial reserve on PCD loans at merger		$ 55,830,000	55,800,000
Financing receivable, allowance for credit loss, excluding accrued interest		$ 40,900,000
Residential Portfolio Segment
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing receivable, collateral dependent loans			1,100,000	800,000
Commercial Portfolio Segment
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing receivable, collateral dependent loans			107,700,000	30,700,000
Unrated | Minimum
Financing Receivable, Allowance for Credit Loss [Line Items]
Lines of credit, exposure			100,000
Residential real estate | Residential Portfolio Segment
Financing Receivable, Allowance for Credit Loss [Line Items]
Accrued interest purchase			0	32,000,000.0
Amortized cost balance of loans purchased			367,000,000.0	385,500,000
Provision for allowance for loan losses			3,241,000	(1,423,000)	7,990,000
Financing receivable, excluding accrued interest, allowance for credit loss, writeoff			28,000	0	0
Initial reserve on PCD loans at merger			2,919,000
Financing receivable, allowance for credit loss, excluding accrued interest			$ 32,291,000	$ 25,954,000	28,129,000		6,556,000
Number of loans in process of foreclosure | loan			4	2
Mortgage loans in process of foreclosure, amount			$ 400,000	$ 200,000
Commercial and industrial | Commercial Portfolio Segment
Financing Receivable, Allowance for Credit Loss [Line Items]
Proceeds from sales of commercial loans held for investment			0	214,200,000
Realized loss on sales of commercial loans				2,700,000
Provision for allowance for loan losses			7,483,000	(230,000)	(3,745,000)
Financing receivable, excluding accrued interest, allowance for credit loss, writeoff			40,000	13,000	269,000
Initial reserve on PCD loans at merger			6,589,000
Financing receivable, allowance for credit loss, excluding accrued interest			41,090,000	26,959,000	26,859,000		18,018,000
Commercial real estate | Commercial Portfolio Segment
Financing Receivable, Allowance for Credit Loss [Line Items]
Provision for allowance for loan losses			45,393,000	18,555,000	8,921,000
Financing receivable, excluding accrued interest, allowance for credit loss, writeoff			42,556,000	8,008,000	0
Initial reserve on PCD loans at merger			45,656,000
Financing receivable, allowance for credit loss, excluding accrued interest			116,175,000	65,475,000	54,730,000		52,373,000
Commercial real estate | Commercial Portfolio Segment | Non-accrual
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing receivable, excluding accrued interest, allowance for credit loss, writeoff			41,300,000
Unfunded Loan Commitment
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing receivable, allowance for credit loss, excluding accrued interest			13,100,000	14,100,000
Business banking | Commercial Portfolio Segment
Financing Receivable, Allowance for Credit Loss [Line Items]
Provision for allowance for loan losses			5,714,000	1,642,000	(731,000)
Financing receivable, excluding accrued interest, allowance for credit loss, writeoff			2,498,000	4,645,000	2,292,000
Initial reserve on PCD loans at merger			581,000
Financing receivable, allowance for credit loss, excluding accrued interest			19,899,000	14,913,000	16,189,000		10,983,000
Business banking | Line of Credit | Unrated | Commercial Portfolio Segment | Maximum
Financing Receivable, Allowance for Credit Loss [Line Items]
Lines of credit, exposure			1,500,000
Consumer home equity | Consumer Portfolio Segment
Financing Receivable, Allowance for Credit Loss [Line Items]
Provision for allowance for loan losses			1,760,000	(692,000)	852,000
Financing receivable, excluding accrued interest, allowance for credit loss, writeoff			59,000	7,000	1,000
Initial reserve on PCD loans at merger			40,000
Financing receivable, allowance for credit loss, excluding accrued interest			$ 7,472,000	$ 5,595,000	6,454,000		3,722,000
Number of loans in process of foreclosure | loan			6	3
Mortgage loans in process of foreclosure, amount			$ 500,000	$ 200,000
Cumulative effect accounting adjustment
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing receivable, allowance for credit loss, excluding accrued interest					(1,143,000)		27,086,000
Cumulative effect accounting adjustment | Residential real estate | Residential Portfolio Segment
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing receivable, allowance for credit loss, excluding accrued interest					(849,000)		13,489,000
Cumulative effect accounting adjustment | Commercial and industrial | Commercial Portfolio Segment
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing receivable, allowance for credit loss, excluding accrued interest					47,000		11,533,000
Cumulative effect accounting adjustment | Commercial real estate | Commercial Portfolio Segment
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing receivable, allowance for credit loss, excluding accrued interest					0		(6,655,000)
Cumulative effect accounting adjustment | Business banking | Commercial Portfolio Segment
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing receivable, allowance for credit loss, excluding accrued interest					(140,000)		6,160,000
Cumulative effect accounting adjustment | Consumer home equity | Consumer Portfolio Segment
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing receivable, allowance for credit loss, excluding accrued interest					$ (201,000)		$ 1,857,000
Federal Home Loan Bank Advances
Financing Receivable, Allowance for Credit Loss [Line Items]
FHLB advances			17,600,000	17,700,000
Federal Reserve Bank Advances
Financing Receivable, Allowance for Credit Loss [Line Items]
Debt, long-term and short-term, combined amount			0	0
Asset Pledged as Collateral | Federal Home Loan Bank Advances
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing receivable, excluding accrued interest, after allowance for credit loss			2,300,000,000	4,600,000,000
Asset Pledged as Collateral | Federal Reserve Bank Advances
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing receivable, excluding accrued interest, after allowance for credit loss			$ 3,100,000,000	$ 1,100,000,000

[1] Excludes the mark-to-market adjustment related to one commercial and industrial loan transferred to held for sale during the year ended December 31, 2023 which is included in the change in loans held for sale.