RETIREMENT BENEFITS - Accumulated Other Comprehensive Income (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Change in accumulated other comprehensive income (loss)
Beginning of year balance, net of tax	$ 206,251	$ 201,838
Change, net of tax	423	(295)	$ 97
End of year balance, net of tax	209,366	206,251	201,838
Citibank Korea Inc.
Change in accumulated other comprehensive income (loss)
Curtailment and settlement benefit	7		36
Benefit plans
Change in accumulated other comprehensive income (loss)
Beginning of year balance, net of tax	(6,050)	(5,755)	(5,852)
Actuarial assumptions changes and plan experience	625	(547)	3,923
Net (loss) gain due to difference between actual and expected returns	(562)	263	(4,225)
Net amortization	239	175	198
Prior service benefit	1	2	0
Curtailment/settlement gain (loss)		(7)	(37)
Foreign exchange impact and other	271	(239)	172
Change in deferred taxes, net	(153)	58	66
Change, net of tax	423	(295)	97
End of year balance, net of tax	$ (5,627)	$ (6,050)	$ (5,755)