DISCONTINUED OPERATIONS, SIGNIFICANT DISPOSALS AND OTHER BUSINESS EXITS - Summarized Reserve Charges (Details) - USD ($) $ in Millions	2 Months Ended	3 Months Ended			12 Months Ended
	Dec. 31, 2021	Dec. 31, 2023	Jun. 30, 2022	Mar. 31, 2022	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Employee termination costs
Beginning balance at January 1, 2023					$ 0
Additional charges (releases)		$ 780			781	$ 0	$ 0
Utilization					(69)
Foreign exchange					0
Ending balance at December 31, 2023		712			712	0
Employee Severance Costs
Employee termination costs
Beginning balance at January 1, 2023					0
Additional charges (releases)					687
Utilization					0
Foreign exchange					0
Ending balance at December 31, 2023		$ 687			$ 687	0
Citibank Korea Inc. | Employee Severance Costs
Employee termination costs
Beginning balance at January 1, 2023	$ 1,052		$ 714	$ 1,054		$ 1,054
Additional charges (releases)			(3)	31
Utilization	(1)		(670)	(347)
Foreign exchange	3		(41)	(24)
Ending balance at December 31, 2023	$ 1,054		$ 0	$ 714			$ 1,054