RETIREMENT BENEFITS - Net (Benefit) Expense (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
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 amounts in AOCI:
Total net expense (benefit)	$ 28	$ 14	$ 11
Citibank Korea Inc.
Amortization of amounts in AOCI:
Curtailment and settlement benefit	7		36
U.S. | Pension Plans
Defined Benefit Plan, Net Periodic Benefit Cost
Service cost	0	0	0
Interest cost on benefit obligation	466	505	442
Expected return on assets	(602)	(640)	(612)
Amortization of amounts in AOCI:
Prior service cost (benefit)	2	2	2
Net actuarial loss (gain)	179	151	162
Curtailment loss (gain)	0	0	0
Settlement loss (gain)	0	0	0
Total net expense (benefit)	45	18	(6)
U.S. | Postretirement Benefit Plans
Defined Benefit Plan, Net Periodic Benefit Cost
Service cost	0	0	0
Interest cost on benefit obligation	16	18	16
Expected return on assets	(11)	(13)	(11)
Amortization of amounts in AOCI:
Prior service cost (benefit)	(9)	(9)	(9)
Net actuarial loss (gain)	(10)	(12)	(9)
Curtailment loss (gain)	0	0	0
Settlement loss (gain)	0	0	0
Total net expense (benefit)	(14)	(16)	(13)
Non-U.S. | Pension Plans
Defined Benefit Plan, Net Periodic Benefit Cost
Service cost	115	115	116
Interest cost on benefit obligation	421	409	329
Expected return on assets	(326)	(327)	(263)
Amortization of amounts in AOCI:
Prior service cost (benefit)	(3)	(5)	(7)
Net actuarial loss (gain)	76	72	58
Curtailment loss (gain)	(3)	(16)	(22)
Settlement loss (gain)	5	9	(15)
Total net expense (benefit)	285	257	196
Non-U.S. | Postretirement Benefit Plans
Defined Benefit Plan, Net Periodic Benefit Cost
Service cost	1	1	2
Interest cost on benefit obligation	107	106	90
Expected return on assets	(80)	(77)	(69)
Amortization of amounts in AOCI:
Prior service cost (benefit)	(7)	(9)	(8)
Net actuarial loss (gain)	10	(18)	6
Curtailment loss (gain)	0	0	0
Settlement loss (gain)	0	0	0
Total net expense (benefit)	$ 31	$ 3	$ 21