Amount of fees and commissions from banking, advisory, brokerage, and securities underwriting activities. Activities include, but are not limited to, underwriting securities, private placements of securities, investment advisory and management services, merger and acquisition services, sale and servicing of mutual funds, and other related consulting fees.

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Business Segments - Narrative (Details) $ in Millions	3 Months Ended		12 Months Ended
Business Segments - Narrative (Details) $ in Millions	Mar. 31, 2024 USD ($) segment	Mar. 31, 2023 USD ($)	Dec. 31, 2023 USD ($)	Dec. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)
Segment Reporting Information
Number of business segments \| segment	3
Other noninterest expense	$ (271)	$ (253)
Wealth and asset management revenue	(161)	(146)
Operating Segments \| Investment Advisors [Member]
Segment Reporting Information
Other noninterest expense	(38)	(36)
Wealth and asset management revenue	$ (101)	$ (92)
Operating Segments \| Investment Advisors [Member] \| Revision of Prior Period, Adjustment
Segment Reporting Information
Other noninterest expense			$ 186	$ 177	$ 180
Wealth and asset management revenue			$ 186	$ 177	$ 180