Derivative Financial Instruments - Schedule of Classification on the balance sheet for the periods indicated (Details) - USD ($) $ in Thousands	Sep. 30, 2024	Jul. 12, 2024	Dec. 31, 2023
Derivative Assets
Derivative Asset, Statement of Financial Position [Extensible Enumeration]	Other assets		Other assets
Derivative Asset	$ 51,475		$ 20,456
Derivative Liabilities
Derivative Liability, Statement of Financial Position [Extensible Enumeration]	Other liabilities		Other liabilities
Net Amount	$ 78,858		$ 63,075
Cambridge Bancorp
Derivative Liabilities
Derivative not designated as hedging instruments, Interest rate swaps assets and liabilities		$ 42,500
Asset Derivatives
Derivative Assets
Derivative Instruments in Hedges, Assets, at Fair Value	395		10
Derivatives not designated as hedging instruments, Interest rate swaps, Other assets	50,476		19,535
Derivatives not designated as hedging instruments, Other assets	51,080		20,446
Asset Derivatives | Risk participation agreements
Derivative Assets
Derivatives not designated as hedging instruments, Other assets	13		151
Asset Derivatives | Foreign currency exchange contracts - matched customer book
Derivative Assets
Derivatives not designated as hedging instruments, Other assets	591		760
Asset Derivatives | Foreign currency exchange contracts - foreign currency loan
Derivative Assets
Derivatives not designated as hedging instruments, Other assets	0		0
Liability Derivatives
Derivative Liabilities
Derivative Instruments in Hedges, Liabilities, at Fair Value	3		893
Derivative not designated as hedging instruments, Interest rate swaps, Other liabilities	78,260		61,217
Derivatives not designated as hedging instruments, Other liabilities	78,855		62,182
Liability Derivatives | Risk participation agreements
Derivative Liabilities
Derivatives not designated as hedging instruments, Other liabilities	11		106
Liability Derivatives | Foreign currency exchange contracts - matched customer book
Derivative Liabilities
Derivatives not designated as hedging instruments, Other liabilities	520		672
Liability Derivatives | Foreign currency exchange contracts - foreign currency loan
Derivative Liabilities
Derivatives not designated as hedging instruments, Other liabilities	$ 64		$ 187