Goodwill and Other Intangible Assets - Gross Carrying Amount, Accumulated Amortization and Net Carrying Amount of Other Intangible Assets (Details) - USD ($) $ in Millions	Mar. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Finite-Lived Intangible Assets [Line Items]
Gross carrying amount	$ 3,915	$ 3,938
Accumulated amortization	(2,657)	(2,618)
Net carrying amount	1,258	1,320	$ 1,544
Client relationships
Finite-Lived Intangible Assets [Line Items]
Gross carrying amount	2,742	2,761
Accumulated amortization	(1,833)	(1,808)
Net carrying amount	909	953
Technology
Finite-Lived Intangible Assets [Line Items]
Gross carrying amount	404	402
Accumulated amortization	(225)	(216)
Net carrying amount	179	186
Core deposits
Finite-Lived Intangible Assets [Line Items]
Gross carrying amount	685	690
Accumulated amortization	(521)	(516)
Net carrying amount	164	174
Other
Finite-Lived Intangible Assets [Line Items]
Gross carrying amount	84	85
Accumulated amortization	(78)	(78)
Net carrying amount	$ 6	$ 7