CHANGES IN ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS) (AOCI) - Schedule of Pre-Tax and After-Tax (Details) - USD ($) $ in Millions	3 Months Ended				6 Months Ended
	Jun. 30, 2023		Jun. 30, 2022		Jun. 30, 2023	Jun. 30, 2022
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period					$ 201,838
Balance, end of period	$ 209,422		$ 199,626		209,422	$ 199,626
Change in net unrealized gains (losses) on debt securities
Change in accumulated other comprehensive income (loss), pretax
Other comprehensive income (loss), pretax	210		(1,990)		1,323	(7,614)
Change in accumulated other comprehensive income (loss), tax effect
Other comprehensive income (loss), tax effect	(84)		489		(361)	1,836
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period	(5,162)		(4,891)		(5,998)	(614)
Total other comprehensive income	126		(1,501)		962	(5,778)
Balance, end of period	(5,036)		(6,392)		(5,036)	(6,392)
Change in net unrealized gains (losses) on debt securities | Cumulative Effect, Period of Adoption, Adjustment
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period					0
Change in net unrealized gains (losses) on debt securities | Cumulative Effect, Period of Adoption, Adjusted Balance
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period					(5,998)
Debt valuation adjustment (DVA)
Change in accumulated other comprehensive income (loss), pretax
Other comprehensive income (loss), pretax	(837)		2,592		(1,270)	3,642
Change in accumulated other comprehensive income (loss), tax effect
Other comprehensive income (loss), tax effect	218		(625)		326	(882)
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period	517		(394)		842	(1,187)
Total other comprehensive income	(619)		1,967		(944)	2,760
Balance, end of period	(102)		1,573		(102)	1,573
Debt valuation adjustment (DVA) | Cumulative Effect, Period of Adoption, Adjustment
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period					0
Debt valuation adjustment (DVA) | Cumulative Effect, Period of Adoption, Adjusted Balance
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period					842
Hedges
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period	(12)		1		8	(47)
Total other comprehensive income	17		9		(3)	57
Balance, end of period	5		10		5	10
Hedges | Cumulative Effect, Period of Adoption, Adjustment
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period					0
Hedges | Cumulative Effect, Period of Adoption, Adjusted Balance
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period					8
Benefit plans
Change in accumulated other comprehensive income (loss), pretax
Other comprehensive income (loss), pretax	(156)		(73)		(312)	104
Change in accumulated other comprehensive income (loss), tax effect
Other comprehensive income (loss), tax effect	20		(16)		72	(22)
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period	(5,859)		(5,681)		(5,755)	(5,852)
Total other comprehensive income	(136)		(89)		(240)	82
Balance, end of period	(5,995)		(5,770)		(5,995)	(5,770)
Benefit plans | Cumulative Effect, Period of Adoption, Adjustment
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period					0
Benefit plans | Cumulative Effect, Period of Adoption, Adjusted Balance
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period					(5,755)
Foreign currency translation adjustment
Change in accumulated other comprehensive income (loss), pretax
Other comprehensive income (loss), pretax	15		(1,414)		803	(1,483)
Change in accumulated other comprehensive income (loss), tax effect
Other comprehensive income (loss), tax effect	8		(216)		61	(161)
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period	(32,796)		(31,180)		(33,637)	(31,166)
Total other comprehensive income	23		(1,630)		864	(1,644)
Balance, end of period	(32,773)		(32,810)		(32,773)	(32,810)
Foreign currency translation adjustment | Cumulative Effect, Period of Adoption, Adjustment
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period					0
Foreign currency translation adjustment | Cumulative Effect, Period of Adoption, Adjusted Balance
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period					(33,637)
Long-duration insurance contracts
Change in accumulated other comprehensive income (loss), pretax
Other comprehensive income (loss), pretax	(8)				(2)
Change in accumulated other comprehensive income (loss), tax effect
Other comprehensive income (loss), tax effect	2				1
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period	32		0		0	0
Total other comprehensive income	(6)		0		(1)	0
Balance, end of period	26		0		26	0
Long-duration insurance contracts | Cumulative Effect, Period of Adoption, Adjustment
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period					27
Long-duration insurance contracts | Cumulative Effect, Period of Adoption, Adjusted Balance
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period					27
Citigroup's accumulated other comprehensive income (loss)
Change in accumulated other comprehensive income (loss), pretax
Balance, beginning of period, pretax	(53,443)		(51,807)		(55,253)	(45,383)
Other comprehensive income (loss), pretax	(521)		(1,759)		1,250	(8,183)
Balance, end of period, pretax	(53,964)		(53,566)		(53,964)	(53,566)
Change in accumulated other comprehensive income (loss), tax effect
Balance, beginning of period, tax effect	8,002		8,222		8,191	6,618
Other comprehensive income (loss), tax effect	97		(151)		(80)	1,453
Balance, end of period, tax effect	8,099		8,071		8,099	8,071
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period	(45,441)		(43,585)		(47,062)	(38,765)
Total other comprehensive income	(424)		(1,910)		1,170	(6,730)
Balance, end of period	(45,865)		(45,495)		(45,865)	(45,495)
Citigroup's accumulated other comprehensive income (loss) | Cumulative Effect, Period of Adoption, Adjustment
Change in accumulated other comprehensive income (loss), pretax
Balance, beginning of period, pretax					39
Change in accumulated other comprehensive income (loss), tax effect
Balance, beginning of period, tax effect					(12)
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period	0	[1]	0	[1]	27	0
Citigroup's accumulated other comprehensive income (loss) | Cumulative Effect, Period of Adoption, Adjusted Balance
Change in accumulated other comprehensive income (loss), pretax
Balance, beginning of period, pretax					(55,214)
Change in accumulated other comprehensive income (loss), tax effect
Balance, beginning of period, tax effect					8,179
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period	(45,441)		(43,585)		(47,035)	(38,765)
Cash flow hedges | Hedges
Change in accumulated other comprehensive income (loss), pretax
Other comprehensive income (loss), pretax	233		(886)		712	(2,908)
Change in accumulated other comprehensive income (loss), tax effect
Other comprehensive income (loss), tax effect	(62)		220		(180)	701
Change in accumulated other comprehensive income (loss), after-tax
Total other comprehensive income	171		(666)		532	(2,207)
Fair value hedges | Hedges
Change in accumulated other comprehensive income (loss), pretax
Other comprehensive income (loss), pretax	22		12		(4)	76
Change in accumulated other comprehensive income (loss), tax effect
Other comprehensive income (loss), tax effect	(5)		(3)		1	(19)
Change in accumulated other comprehensive income (loss), after-tax
Total other comprehensive income	$ 17		9		$ (3)	57
Fair value hedges | Long-duration insurance contracts
Change in accumulated other comprehensive income (loss), pretax
Other comprehensive income (loss), pretax			0			0
Change in accumulated other comprehensive income (loss), tax effect
Other comprehensive income (loss), tax effect			$ 0			$ 0

[1] See Note 1 for additional details.