Acquisition and Divestitures - Summary of Merger Related Expenses (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outside data processing and software	$ 492	$ 437	$ 376
Advertising and marketing	$ 104	$ 108	90
Peoples United Financial Inc
Accounts, Notes, Loans and Financing Receivable [Line Items]
Salaries and employee benefits			102
Equipment and net occupancy			7
Outside data processing and software			5
Professional and other services			72
Advertising and marketing			9
Other cost of operations			143
Other Expenses, Total			$ 338