Subsequent Events (Details) $ / shares in Units, $ in Thousands	Jul. 12, 2024 USD ($) office $ / shares shares	Jun. 30, 2024 USD ($)	Dec. 31, 2023 USD ($)
Subsequent Event [Line Items]
Total assets		$ 21,044,169	$ 21,133,278
Deposits		$ 17,537,809	$ 17,596,217
Subsequent Event | Cambridge Bancorp
Subsequent Event [Line Items]
Common stock exchanged (in shares) | shares	4.956
Common stock issued (in shares) | shares	38,900,000
Common stock, par value (in dollars per share) | $ / shares	$ 14.87
Common stock	$ 580,600
Total assets	5,000,000
Securities sold	900,000
Gain (loss) on sale of investments	$ 0
Subsequent Event | Cambridge Trust
Subsequent Event [Line Items]
Number of full service banking offices | office	19
Total assets	$ 5,300,000
Deposits	$ 3,900,000