CHANGES IN ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS) (AOCI) - Reclassification out of AOCI (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Pretax and after-tax amounts reclassified out of accumulated other comprehensive income (loss)
Realized (gains) losses on sales of investments	$ (304)	$ (361)	$ (1,484)	$ (959)
Tax effect	815	1,079	1,522	3,727
Income (loss) from continuing operations	(3,261)	(4,943)	(7,112)	(14,472)
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	(210)	(259)	(1,052)	(697)
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 on sales of investments	(304)	(361)	(1,484)	(959)
Gross impairment losses	30	13	101	18
Income from continuing operations before income taxes	(274)	(348)	(1,383)	(941)
Tax effect	64	89	331	244
Income (loss) from continuing operations	(210)	(259)	(1,052)	(697)
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	6	(5)	2	16
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 on sales of investments	8	(6)	3	21
Tax effect	(2)	1	(1)	(5)
Income (loss) from continuing operations	6	(5)	2	16
Cash flow hedges
Pretax and after-tax amounts reclassified out of accumulated other comprehensive income (loss)
Total amounts reclassified out of AOCI, after-tax	(194)	81	(347)	284
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	(255)	98	(456)	366
Tax effect	61	(17)	109	(82)
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	(256)	96	(459)	360
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	2	3	6
Pension liability adjustments
Pretax and after-tax amounts reclassified out of accumulated other comprehensive income (loss)
Total amounts reclassified out of AOCI, pretax	76	66	227	196
Total tax effect	(19)	(16)	(57)	(48)
Total amounts reclassified out of AOCI, after-tax	57	50	170	148
Prior service cost (benefit)
Pretax and after-tax amounts reclassified out of accumulated other comprehensive income (loss)
Total amounts reclassified out of AOCI, pretax	(4)	(3)	(10)	(9)
Net actuarial loss
Pretax and after-tax amounts reclassified out of accumulated other comprehensive income (loss)
Total amounts reclassified out of AOCI, pretax	85	73	239	207
Curtailment/settlement impact
Pretax and after-tax amounts reclassified out of accumulated other comprehensive income (loss)
Total amounts reclassified out of AOCI, pretax	(5)	(4)	(2)	(2)
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	0	0	0	0
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	0	0	0	0
Tax effect	0	0	0	0
Total amounts reclassified out of AOCI, after-tax	0	0	0	0
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	0	0	0	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	0	0	0	0
Tax effect	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	(445)	(190)	(1,609)	(358)
Total tax effect	104	57	382	109
Total amounts reclassified out of AOCI, after-tax	$ (341)	$ (133)	$ (1,227)	$ (249)