Goodwill and Other Intangible Assets - Summary of Goodwill by Reportable Segment (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2024	Dec. 31, 2023
Goodwill [Roll Forward]
Goodwill, beginning balance	$ 1,510	$ 1,511
Additions	0	0
Divestitures	0	(1)
Goodwill, ending balance	1,510	1,510
Regional Banking
Goodwill [Roll Forward]
Goodwill, beginning balance	825	825
Additions	0	0
Divestitures	0	0
Goodwill, ending balance	825	825
Specialty Banking
Goodwill [Roll Forward]
Goodwill, beginning balance	685	686
Additions	0	0
Divestitures	0	(1)
Goodwill, ending balance	$ 685	$ 685