RESTRUCTURING - Schedule of Restructuring Reserve Activity (Details) - USD ($) $ in Millions	3 Months Ended						9 Months Ended				12 Months Ended
	Sep. 30, 2025		Jun. 30, 2025	Mar. 31, 2025	Sep. 30, 2024	[1]	Sep. 30, 2025		Sep. 30, 2024		Dec. 31, 2024	Dec. 31, 2023
Restructuring Reserve [Roll Forward]
Beginning of period	$ 11		$ 20	$ 42			$ 42		$ 712		$ 712	$ 0
Restructuring charges	0		0	1							408	781
Change in estimate	(5)		(2)	(4)							(149)	0
Net restructuring charges	(5)	[1]	(2)	(3)	$ 9		(10)	[1]	270	[1]	259	781
Payments and utilization	(2)		(8)	(13)							(936)	(69)
Foreign exchange	0		1	(6)							7	0
End of period	4		11	20			4				42	712
Personnel costs
Restructuring Reserve [Roll Forward]
Beginning of period	11		20	42			42		687		687	0
Restructuring charges	0		0	1							354	687
Change in estimate	(5)		(2)	(4)							(146)	0
Net restructuring charges	(5)		(2)	(3)							208	687
Payments and utilization	(2)		(8)	(13)							(860)	0
Foreign exchange	0		1	(6)							7	0
End of period	4		11	20			4				42	687
Other
Restructuring Reserve [Roll Forward]
Beginning of period	0		0	0			0		$ 25		25	0
Restructuring charges	0		0	0							54	94
Change in estimate	0		0	0							(3)	0
Net restructuring charges	0		0	0							51	94
Payments and utilization	0		0	0							(76)	(69)
Foreign exchange	0		0	0							0	0
End of period	$ 0		$ 0	$ 0			$ 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.