RETIREMENT BENEFITS - Net (Benefit) Expense (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
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 unrecognized
Total net expense (benefit)	$ 11	$ 10	$ 9
Citibank Korea Inc.
Amortization of unrecognized
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	442	351	378
Expected return on assets	(612)	(683)	(824)
Amortization of unrecognized
Prior service cost (benefit)	2	2	2
Net actuarial loss (gain)	162	228	233
Curtailment loss (gain)	0	0	0
Settlement loss (gain)	0	0	0
Total net expense (benefit)	(6)	(102)	(211)
U.S. | Postretirement Benefit Plans
Defined Benefit Plan, Net Periodic Benefit Cost
Service cost	0	0	0
Interest cost on benefit obligation	16	13	17
Expected return on assets	(11)	(13)	(17)
Amortization of unrecognized
Prior service cost (benefit)	(9)	(9)	(2)
Net actuarial loss (gain)	(9)	(3)	0
Curtailment loss (gain)	0	0	0
Settlement loss (gain)	0	0	0
Total net expense (benefit)	(13)	(12)	(2)
Non-U.S. | Pension Plans
Defined Benefit Plan, Net Periodic Benefit Cost
Service cost	116	149	147
Interest cost on benefit obligation	329	268	246
Expected return on assets	(263)	(253)	(245)
Amortization of unrecognized
Prior service cost (benefit)	(7)	(6)	5
Net actuarial loss (gain)	58	62	70
Curtailment loss (gain)	(22)	1	(8)
Settlement loss (gain)	(15)	10	(1)
Total net expense (benefit)	196	231	214
Non-U.S. | Postretirement Benefit Plans
Defined Benefit Plan, Net Periodic Benefit Cost
Service cost	2	6	7
Interest cost on benefit obligation	90	96	93
Expected return on assets	(69)	(84)	(77)
Amortization of unrecognized
Prior service cost (benefit)	(8)	(9)	(9)
Net actuarial loss (gain)	6	13	20
Curtailment loss (gain)	0	0	0
Settlement loss (gain)	0	0	0
Total net expense (benefit)	$ 21	$ 22	$ 34