Acquisition and Divestitures - Summary of Merger Related Expenses (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outside data processing and software	$ 437	$ 376	$ 292
Professional and other services	413	509	379
Advertising and marketing	$ 108	90	64
Peoples United Financial Inc [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Salaries and employee benefits		102
Equipment and net occupancy		7
Outside data processing and software		5	1
Professional and other services		72	37
Advertising and marketing		9	1
Other cost of operations		143	5
Other Expenses, Total		$ 338	$ 44