CHANGES IN ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS) (AOCI) - Reclassification out of AOCI (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Pretax and after-tax amounts reclassified out of accumulated other comprehensive income (loss)
Realized gains and (losses)	$ 23	$ 49	$ 138	$ 121
Impairment losses on investments	(17)	(71)	(47)	(157)
Income from continuing operations before income taxes	4,310	4,042	8,854	10,225
Tax effect	(1,047)	(1,090)	(2,183)	(2,621)
Income (loss) from continuing operations	3,263	2,952	6,671	7,604
Subtotal, pretax	62	(92)	539	821
Other Noninterest Expense	(3,314)	(2,917)	(6,552)	(5,612)
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, after-tax	228	388	461	750
Citigroup's accumulated other comprehensive income (loss) | (Gain) loss reclassified from AOCI
Pretax and after-tax amounts reclassified out of accumulated other comprehensive income (loss)
Total amounts reclassified out of AOCI, after-tax	228	388	461	750
Total tax effect	(81)	(129)	(155)	(251)
Total amounts reclassified out of AOCI, pretax	309	517	616	1,001
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	(14)	(7)	(90)	(26)
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)	(23)	(49)	(138)	(121)
Impairment losses on investments	9	43	23	94
Income from continuing operations before income taxes	(14)	(6)	(115)	(27)
Tax effect	0	(1)	25	1
Income (loss) from continuing operations	(14)	(7)	(90)	(26)
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	2	(6)	12	(4)
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)	3	(7)	16	(4)
Tax effect	(1)	1	(4)	0
Income (loss) from continuing operations	2	(6)	12	(4)
Cash flow hedges
Pretax and after-tax amounts reclassified out of accumulated other comprehensive income (loss)
Total amounts reclassified out of AOCI, after-tax	198	377	458	732
Cash flow hedges | (Gain) loss reclassified from AOCI
Pretax and after-tax amounts reclassified out of accumulated other comprehensive income (loss)
Tax effect	(63)	(119)	(146)	(234)
Subtotal, pretax	261	496	604	966
Total amounts reclassified out of AOCI, after-tax	198	377	458	732
Cash flow hedges | (Gain) loss reclassified from AOCI | Interest rate contracts
Pretax and after-tax amounts reclassified out of accumulated other comprehensive income (loss)
Subtotal, pretax	260	495	602	964
Cash flow hedges | (Gain) loss reclassified from AOCI | Foreign exchange contracts
Pretax and after-tax amounts reclassified out of accumulated other comprehensive income (loss)
Subtotal, pretax	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, after-tax	44	34	93	62
Benefit plan adjustments | (Gain) loss reclassified from AOCI
Pretax and after-tax amounts reclassified out of accumulated other comprehensive income (loss)
Other Noninterest Expense	61	47	126	85
Total amounts reclassified out of AOCI, after-tax	44	34	93	62
Total tax effect	(17)	(13)	(33)	(23)
Prior service cost (benefit) | (Gain) loss reclassified from AOCI
Pretax and after-tax amounts reclassified out of accumulated other comprehensive income (loss)
Other Noninterest Expense	(5)	(5)	(10)	(11)
Net actuarial loss | (Gain) loss reclassified from AOCI
Pretax and after-tax amounts reclassified out of accumulated other comprehensive income (loss)
Other Noninterest Expense	64	51	134	100
Curtailment/settlement impact | (Gain) loss reclassified from AOCI
Pretax and after-tax amounts reclassified out of accumulated other comprehensive income (loss)
Other Noninterest Expense	2	1	2	(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	(1)	(10)	(11)	(14)
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	(1)	(13)	(14)	(19)
Tax effect	0	3	3	5
Total amounts reclassified out of AOCI, after-tax	(1)	(10)	(11)	(14)
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	0	0
Tax effect	0	0	0	0
Total amounts reclassified out of AOCI, after-tax	0	0	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	(1)		(1)
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	(1)	0	(1)	0
Tax effect	0	0	0	0
Total amounts reclassified out of AOCI, after-tax	$ (1)	$ 0	$ (1)	$ 0