CHANGES IN ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS) (AOCI) - Reclassification out of AOCI (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021
Pretax and after-tax amounts reclassified out of accumulated other comprehensive income (loss)
Realized (gains) losses	$ 58	$ (137)	$ (22)	$ (538)
Tax effect	1,182	1,155	2,123	3,487
Income (loss) from continuing operations	(4,789)	(6,193)	(9,114)	(14,170)
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	111	(95)	117	(361)
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	58	(137)	(22)	(538)
Gross impairment losses	90	9	180	78
Income from continuing operations before income taxes	148	(128)	158	(460)
Tax effect	(37)	33	(41)	99
Income (loss) from continuing operations	111	(95)	117	(361)
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	(1)	10	(1)	52
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	(1)	13	(1)	69
Tax effect	0	(3)	0	(17)
Income (loss) from continuing operations	(1)	10	(1)	52
Cash flow hedges
Pretax and after-tax amounts reclassified out of accumulated other comprehensive income (loss)
Total amounts reclassified out of AOCI, after-tax	(151)	(201)	(368)	(413)
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	(198)	(265)	(483)	(542)
Tax effect	47	64	115	129
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	(199)	(266)	(485)	(544)
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	2	2
Benefit plan adjustments
Pretax and after-tax amounts reclassified out of accumulated other comprehensive income (loss)
Total amounts reclassified out of AOCI, pretax	236	69	84	150
Total tax effect	(54)	(18)	(23)	(38)
Total amounts reclassified out of AOCI, after-tax	182	51	61	112
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)	(11)	(12)
Net actuarial loss
Pretax and after-tax amounts reclassified out of accumulated other comprehensive income (loss)
Total amounts reclassified out of AOCI, pretax	58	71	128	158
Curtailment/settlement impact
Pretax and after-tax amounts reclassified out of accumulated other comprehensive income (loss)
Total amounts reclassified out of AOCI, pretax	183	4	(33)	4
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	5	1	7	1
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	7	1	10	1
Tax effect	(2)	0	(3)	0
Total amounts reclassified out of AOCI, after-tax	5	1	7	1
Foreign currency translation adjustment
Pretax and after-tax amounts reclassified out of accumulated other comprehensive income (loss)
Total amounts reclassified out of AOCI, after-tax	345	0	345	0
Foreign currency translation adjustment | (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	397	0	397	0
Tax effect	(52)	0	(52)	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	589	(310)	165	(782)
Total tax effect	(98)	76	(4)	173
Total amounts reclassified out of AOCI, after-tax	$ 491	$ (234)	$ 161	$ (609)