CHANGES IN ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS) (AOCI) - Reclassification out of AOCI (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Pretax and after-tax amounts reclassified out of accumulated other comprehensive income (loss)
Realized gains and (losses)	$ (115)	$ (72)
Gross impairment losses	30	86
Tax effect	1,136	1,531
Income (loss) from continuing operations	(3,408)	(4,652)
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	(76)	(19)
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 and (losses)	(115)	(72)
Gross impairment losses	14	51
Income from continuing operations before income taxes	(101)	(21)
Tax effect	25	2
Income (loss) from continuing operations	(76)	(19)
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	10	2
Debt valuation adjustment (DVA) | (Gain) loss reclassified from AOCI
Pretax and after-tax amounts reclassified out of accumulated other comprehensive income (loss)
Realized gains and (losses)	13	3
Tax effect	(3)	(1)
Income (loss) from continuing operations	10	2
Cash flow hedges
Pretax and after-tax amounts reclassified out of accumulated other comprehensive income (loss)
Total amounts reclassified out of AOCI, after-tax	260	355
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	343	470
Tax effect	(83)	(115)
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	342	469
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
Benefit plan adjustments
Pretax and after-tax amounts reclassified out of accumulated other comprehensive income (loss)
Total amounts reclassified out of AOCI, pretax	63	38
Total tax effect	(16)	(10)
Total amounts reclassified out of AOCI, after-tax	47	28
Prior service cost (benefit)
Pretax and after-tax amounts reclassified out of accumulated other comprehensive income (loss)
Total amounts reclassified out of AOCI, pretax	(5)	(6)
Net actuarial loss
Pretax and after-tax amounts reclassified out of accumulated other comprehensive income (loss)
Total amounts reclassified out of AOCI, pretax	68	49
Curtailment/settlement impact
Pretax and after-tax amounts reclassified out of accumulated other comprehensive income (loss)
Total amounts reclassified out of AOCI, pretax	0	(5)
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	(10)	(4)
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	(13)	(6)
Tax effect	3	2
Total amounts reclassified out of AOCI, after-tax	(10)	(4)
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	0
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	0
Tax effect	0	0
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
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
Tax effect	0	0
Total amounts reclassified out of AOCI, after-tax	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	305	484
Total tax effect	(74)	(122)
Total amounts reclassified out of AOCI, after-tax	$ 231	$ 362