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	Mar. 31, 2024	Dec. 31, 2023	Mar. 31, 2023	Jun. 30, 2022	Mar. 31, 2022	Mar. 31, 2024	Dec. 31, 2023
Employee termination costs
Beginning of period		$ 712		$ 0				$ 0
Additional charges		225	$ 781	0			$ 1,000	781
Foreign exchange		0						0
End of period		764	712				764	712
Severance
Employee termination costs
Beginning of period		687		$ 0				0
Additional charges		171						687
Foreign exchange		0						0
End of period		$ 731	$ 687				$ 731	$ 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