Mergers and Acquisitions - Narrative (Details)		12 Months Ended
	Jul. 12, 2024 USD ($) property lease_obligation branch $ / shares shares	Dec. 31, 2024 USD ($)	Dec. 31, 2023 USD ($)	Dec. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)
Business Acquisition [Line Items]
Goodwill		$ 914,957,000	$ 557,635,000	$ 557,635,000
Initial reserve on PCD loans at merger		55,830,000
Allowance for loan losses		228,952,000	148,993,000	$ 142,211,000	$ 97,787,000
Cambridge Bancorp
Business Acquisition [Line Items]
Common stock exchanged (in shares) | shares	4.956
Issuance of restricted stock awards (in shares) | shares	38,900,000
Common stock, par value (in dollars per share) | $ / shares	$ 14.87
Common stock	$ 580,600,000
Goodwill	357,322,000
Investment securities discount acquired	158,900,000
Gain (loss) on sale of investments	0
Fair value estimated discount	277,000,000.0
Initial reserve on PCD loans at merger	55,830,000	55,800,000
Allowance for loan losses	$ 40,900,000
Number of branches acquired | branch	18
Number of corporate properties acquired | property	2
Number of lease obligations | lease_obligation	23
Right-of-use asset acquired	$ 25,500,000
Lease liability acquired	32,000,000.0
Intangible assets	141,200,000
Business acquisition, goodwill, expected tax deductible amount	0
Bank owned life insurance	35,676,000
Certificate of deposit discount	1,600,000
Escrow deposits of borrowers	3,633,000
Acquisition-related and professional fee costs		$ 36,664,000	$ 5,495,000
Cambridge Bancorp | Core Deposits
Business Acquisition [Line Items]
Intangible assets	$ 115,000,000.0
Weighted average useful life acquired	7 years
Cambridge Bancorp | Customer list intangible
Business Acquisition [Line Items]
Intangible assets	$ 25,000,000.0
Weighted average useful life acquired	11 years
Cambridge Bancorp | Trade name intangible
Business Acquisition [Line Items]
Intangible assets	$ 1,200,000
Weighted average useful life acquired	5 years