DISCONTINUED OPERATIONS, SIGNIFICANT DISPOSALS AND OTHER BUSINESS EXITS - Summarized Reserve Charges (Details) - Citibank Korea Inc. - Employee Severance Costs - USD ($) $ in Millions	2 Months Ended	3 Months Ended
	Dec. 31, 2021	Mar. 31, 2022
Employee termination costs
Beginning of period	$ 1,052	$ 1,054
Additional charges		31
Utilization	(1)	(347)
Foreign exchange	3	(24)
End of period	$ 1,054	$ 714