RETIREMENT BENEFITS - Net (Benefit) Expense (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Defined Benefit Plan Disclosure
Percentage of the significant plans over global pension and postretirement liabilities, which utilize quarterly measurement policy	90.00%
Net (benefit) expense, location	Other operating	Other operating	Other operating
Amortization of:
Total net expense (benefit)	$ 14	$ 11	$ 10
Citibank Korea Inc.
Amortization of:
Curtailment and settlement benefit		36
U.S. | Pension Plans
Defined Benefit Plan, Net Periodic Benefit Cost
Service cost	0	0	0
Interest cost on benefit obligation	505	442	351
Expected return on assets	(640)	(612)	(683)
Amortization of:
Prior service cost (benefit)	2	2	2
Net actuarial loss (gain)	151	162	228
Curtailment loss (gain)	0	0	0
Settlement loss (gain)	0	0	0
Total net expense (benefit)	18	(6)	(102)
U.S. | Postretirement Benefit Plans
Defined Benefit Plan, Net Periodic Benefit Cost
Service cost	0	0	0
Interest cost on benefit obligation	18	16	13
Expected return on assets	(13)	(11)	(13)
Amortization of:
Prior service cost (benefit)	(9)	(9)	(9)
Net actuarial loss (gain)	(12)	(9)	(3)
Curtailment loss (gain)	0	0	0
Settlement loss (gain)	0	0	0
Total net expense (benefit)	(16)	(13)	(12)
Non-U.S. | Pension Plans
Defined Benefit Plan, Net Periodic Benefit Cost
Service cost	115	116	149
Interest cost on benefit obligation	409	329	268
Expected return on assets	(327)	(263)	(253)
Amortization of:
Prior service cost (benefit)	(5)	(7)	(6)
Net actuarial loss (gain)	72	58	62
Curtailment loss (gain)	(16)	(22)	1
Settlement loss (gain)	9	(15)	10
Total net expense (benefit)	257	196	231
Non-U.S. | Postretirement Benefit Plans
Defined Benefit Plan, Net Periodic Benefit Cost
Service cost	1	2	6
Interest cost on benefit obligation	106	90	96
Expected return on assets	(77)	(69)	(84)
Amortization of:
Prior service cost (benefit)	(9)	(8)	(9)
Net actuarial loss (gain)	(18)	6	13
Curtailment loss (gain)	0	0	0
Settlement loss (gain)	0	0	0
Total net expense (benefit)	$ 3	$ 21	$ 22