RESTRUCTURING - Schedule of Restructuring Reserve Activity (Details) - USD ($) $ in Millions	3 Months Ended				12 Months Ended
	Mar. 31, 2025		Mar. 31, 2024		Dec. 31, 2024	Dec. 31, 2023
Restructuring Reserve [Roll Forward]
Beginning of period	$ 42		$ 712		$ 712	$ 0
Restructuring charges	1				408	781
Change in estimate	(4)				(149)
Net restructuring charges	(3)	[1]	225	[1]	259
Payments and utilization	(13)				(936)	(69)
Foreign exchange	(6)				7	0
End of period	20				42	712
Personnel costs
Restructuring Reserve [Roll Forward]
Beginning of period	42		687		687	0
Restructuring charges	1				354	687
Change in estimate	(4)				(146)
Net restructuring charges	(3)				208
Payments and utilization	(13)				(860)	0
Foreign exchange	(6)				7	0
End of period	20				42	687
Other
Restructuring Reserve [Roll Forward]
Beginning of period	0		$ 25		25	0
Restructuring charges	0				54	94
Change in estimate	0				(3)
Net restructuring charges	0				51
Payments and utilization	0				(76)	(69)
Foreign exchange	0				0	0
End of period	$ 0				$ 0	$ 25

[1]     Effective January 1, 2025, certain transaction processing fees paid by Citi, primarily to credit card networks, which were previously presented within Other operating expenses, are presented as contra-revenue within Commissions and fees reported in Non-interest revenue. Prior periods were conformed to reflect this change in presentation.