CHANGES IN ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS) (AOCI) - Reclassification out of AOCI (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Pretax and after-tax amounts reclassified out of accumulated other comprehensive income (loss)
Realized (gains) losses	$ (30)	$ (52)	$ (151)	$ (74)
Gross impairment losses	70	91	227	277
Tax effect	1,203	879	3,824	3,002
Income (loss) from continuing operations	(3,585)	(3,515)	(11,189)	(12,629)
Realized gains (losses) on investment securities
Pretax and after-tax amounts reclassified out of accumulated other comprehensive income (loss)
Total amounts reclassified out of AOCI, after-tax	7	15	(19)	132
Realized gains (losses) on investment securities | (Gain) loss reclassified from AOCI
Pretax and after-tax amounts reclassified out of accumulated other comprehensive income (loss)
Realized (gains) losses	(30)	(52)	(151)	(74)
Gross impairment losses	43	74	137	254
Income from continuing operations before income taxes	13	22	(14)	180
Tax effect	(6)	(7)	(5)	(48)
Income (loss) from continuing operations	7	15	(19)	132
Debt valuation adjustment (DVA)
Pretax and after-tax amounts reclassified out of accumulated other comprehensive income (loss)
Total amounts reclassified out of AOCI, after-tax	9	(2)	5	(3)
Debt valuation adjustment (DVA) | (Gain) loss reclassified from AOCI
Pretax and after-tax amounts reclassified out of accumulated other comprehensive income (loss)
Realized (gains) losses	12	(3)	8	(4)
Tax effect	(3)	1	(3)	1
Income (loss) from continuing operations	9	(2)	5	(3)
Cash flow hedges
Pretax and after-tax amounts reclassified out of accumulated other comprehensive income (loss)
Total amounts reclassified out of AOCI, after-tax	365	107	1,097	(261)
Cash flow hedges | (Gain) loss reclassified from AOCI
Pretax and after-tax amounts reclassified out of accumulated other comprehensive income (loss)
Income from continuing operations before income taxes	481	142	1,447	(341)
Tax effect	(116)	(35)	(350)	80
Cash flow hedges | (Gain) loss reclassified from AOCI | Interest rate contracts
Pretax and after-tax amounts reclassified out of accumulated other comprehensive income (loss)
Income from continuing operations before income taxes	480	141	1,444	(344)
Cash flow hedges | (Gain) loss reclassified from AOCI | Foreign exchange contracts
Pretax and after-tax amounts reclassified out of accumulated other comprehensive income (loss)
Income from continuing operations before income taxes	1	1	3	3
Benefit plan adjustments
Pretax and after-tax amounts reclassified out of accumulated other comprehensive income (loss)
Total amounts reclassified out of AOCI, pretax	51	43	136	127
Total tax effect	(13)	(11)	(36)	(34)
Total amounts reclassified out of AOCI, after-tax	38	32	100	93
Prior service cost (benefit)
Pretax and after-tax amounts reclassified out of accumulated other comprehensive income (loss)
Total amounts reclassified out of AOCI, pretax	(6)	(6)	(17)	(17)
Net actuarial loss
Pretax and after-tax amounts reclassified out of accumulated other comprehensive income (loss)
Total amounts reclassified out of AOCI, pretax	52	49	152	177
Curtailment/settlement impact
Pretax and after-tax amounts reclassified out of accumulated other comprehensive income (loss)
Total amounts reclassified out of AOCI, pretax	5	0	1	(33)
Excluded component of fair value hedges
Pretax and after-tax amounts reclassified out of accumulated other comprehensive income (loss)
Total amounts reclassified out of AOCI, after-tax	(9)	(1)	(23)	6
Excluded component of fair value hedges | (Gain) loss reclassified from AOCI
Pretax and after-tax amounts reclassified out of accumulated other comprehensive income (loss)
Income from continuing operations before income taxes	(12)	(1)	(31)	9
Tax effect	3	0	8	(3)
Total amounts reclassified out of AOCI, after-tax	(9)	(1)	(23)	6
Foreign currency translation adjustment (CTA)
Pretax and after-tax amounts reclassified out of accumulated other comprehensive income (loss)
Total amounts reclassified out of AOCI, after-tax	0	24	0	369
Foreign currency translation adjustment (CTA) | (Gain) loss reclassified from AOCI
Pretax and after-tax amounts reclassified out of accumulated other comprehensive income (loss)
Income from continuing operations before income taxes	0	26	0	423
Tax effect	0	(2)	0	(54)
Long-duration insurance contracts
Pretax and after-tax amounts reclassified out of accumulated other comprehensive income (loss)
Total amounts reclassified out of AOCI, after-tax	0	0	0	0
Long-duration insurance contracts | (Gain) loss reclassified from AOCI
Pretax and after-tax amounts reclassified out of accumulated other comprehensive income (loss)
Income from continuing operations before income taxes	0	0	0	0
Tax effect	0	0	0	0
Total amounts reclassified out of AOCI, after-tax	0	0	0	0
Citigroup's accumulated other comprehensive income (loss)
Pretax and after-tax amounts reclassified out of accumulated other comprehensive income (loss)
Total amounts reclassified out of AOCI, pretax	545	229	1,546	394
Total tax effect	(135)	(54)	(386)	(58)
Total amounts reclassified out of AOCI, after-tax	$ 410	$ 175	$ 1,160	$ 336