DISCONTINUED OPERATIONS, SIGNIFICANT DISPOSALS AND OTHER BUSINESS EXITS - Summarized Reserve Charges (Details) - USD ($) $ in Millions	2 Months Ended	3 Months Ended						6 Months Ended		12 Months Ended
	Dec. 31, 2021	Jun. 30, 2024	Mar. 31, 2024	Dec. 31, 2023	Jun. 30, 2023	Jun. 30, 2022	Mar. 31, 2022	Jun. 30, 2024	Jun. 30, 2023	Dec. 31, 2023
Employee termination costs
Beginning of period		$ 764	$ 712					$ 712	$ 0	$ 0
Additional charges		36	225	$ 781	$ 0			261	0	781
Foreign exchange		(1)	0							0
End of period		272	764	712				272		712
Severance
Employee termination costs
Beginning of period		731	687					687	$ 0	0
Additional charges		39	171							687
Foreign exchange		(1)	0							0
End of period		$ 272	$ 731	$ 687				$ 272		$ 687
Citibank Korea Inc. | Severance
Employee termination costs
Beginning of period	$ 1,052					$ 714	$ 1,054
Additional charges						(3)	31
Utilization	(1)					(670)	(347)
Foreign exchange	3					(41)	(24)
End of period	$ 1,054					$ 0	$ 714