RETIREMENT BENEFITS - Accumulated Other Comprehensive Income (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Change in accumulated other comprehensive income (loss)
Beginning of year balance, net of tax		$ 201,838	$ 202,672
Change, net of tax	[1],[2]	(295)	97	$ 1,012
End of year balance, net of tax		206,251	201,838	202,672
Citibank Korea Inc.
Change in accumulated other comprehensive income (loss)
Curtailment and settlement benefit			36
Benefit plans
Change in accumulated other comprehensive income (loss)
Beginning of year balance, net of tax		(5,755)	(5,852)	(6,864)
Actuarial assumptions changes and plan experience		(547)	3,923	963
Net asset gain (loss) due to difference between actual and expected returns		263	(4,225)	(148)
Net amortization		175	198	280
Prior service benefit (cost)		2	0	(7)
Curtailment/settlement gain (loss)		(7)	(37)	11
Foreign exchange impact and other		(239)	172	153
Change in deferred taxes, net		58	66	(240)
Change, net of tax		(295)	97	1,012
End of year balance, net of tax		$ (6,050)	$ (5,755)	$ (5,852)

[1] See Note 21.

[2] See Note 8.