CHANGES IN ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS) (AOCI) - Schedule of Pre-Tax and After-Tax (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period				$ 206,251
Balance, end of period		$ 209,902	$ 210,195	209,902	$ 210,195
Citigroup's accumulated other comprehensive income (loss)
Change in accumulated other comprehensive income (loss), pretax
Balance, beginning of period, pretax		(54,102)	(53,964)	(52,422)	(55,253)
Other comprehensive income (loss), pretax		2,099	(3)	419	1,247
Balance, end of period, pretax		(52,003)	(53,967)	(52,003)	(53,967)
Change in accumulated other comprehensive income (loss), tax effect
Balance, beginning of period, tax effect		7,425	8,099	7,622	8,191
Other comprehensive income (loss), tax effect		(619)	(309)	(816)	(389)
Balance, end of period, tax effect		6,806	7,790	6,806	7,790
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period		(46,677)	(45,865)	(44,800)	(47,062)
Total other comprehensive income		1,480	(312)	(397)	858
Balance, end of period		(45,197)	(46,177)	(45,197)	(46,177)
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	[1]				27
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		(46,677)	(45,865)	(44,800)	(47,035)
Change in net unrealized gains (losses) on debt securities
Change in accumulated other comprehensive income (loss), pretax
Other comprehensive income (loss), pretax		1,781	(227)	1,853	1,095
Change in accumulated other comprehensive income (loss), tax effect
Other comprehensive income (loss), tax effect		(446)	58	(456)	(302)
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period		(3,682)	(5,036)	(3,744)	(5,998)
Total other comprehensive income		1,335	(169)	1,397	793
Balance, end of period		(2,347)	(5,205)	(2,347)	(5,205)
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		(201)	395	(608)	(875)
Change in accumulated other comprehensive income (loss), tax effect
Other comprehensive income (loss), tax effect		51	(96)	151	230
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period		(1,016)	(102)	(709)	842
Total other comprehensive income		(150)	299	(457)	(645)
Balance, end of period		(1,166)	197	(1,166)	197
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
Excluded component of fair value hedges
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period		(39)	5	(40)	8
Total other comprehensive income		(9)	(12)	(8)	(15)
Balance, end of period		(48)	(7)	(48)	(7)
Excluded component of fair value 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		88	380	405	68
Change in accumulated other comprehensive income (loss), tax effect
Other comprehensive income (loss), tax effect		(39)	(68)	(100)	4
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period		(5,794)	(5,995)	(6,050)	(5,755)
Total other comprehensive income		49	312	305	72
Balance, end of period		(5,745)	(5,683)	(5,745)	(5,683)
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 (CTA)
Change in accumulated other comprehensive income (loss), pretax
Other comprehensive income (loss), pretax		638	(1,532)	(2,071)	(728)
Change in accumulated other comprehensive income (loss), tax effect
Other comprehensive income (loss), tax effect		(222)	36	(201)	96
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period		(35,573)	(32,773)	(32,885)	(33,637)
Total other comprehensive income		416	(1,496)	(2,272)	(632)
Balance, end of period		(35,157)	(34,269)	(35,157)	(34,269)
Foreign currency translation adjustment (CTA) | 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		(26)	33	9	31
Change in accumulated other comprehensive income (loss), tax effect
Other comprehensive income (loss), tax effect		9	(10)	(4)	(9)
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period		56	26	34	0
Total other comprehensive income		(17)	23	5	22
Balance, end of period		39	49	39	49
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
Cash flow hedges | Excluded component of fair value hedges
Change in accumulated other comprehensive income (loss), pretax
Other comprehensive income (loss), pretax		(171)	958	843	1,670
Change in accumulated other comprehensive income (loss), tax effect
Other comprehensive income (loss), tax effect		27	(227)	(210)	(407)
Change in accumulated other comprehensive income (loss), after-tax
Total other comprehensive income		(144)	731	633	1,263
Excluded component of fair value hedges | Excluded component of fair value hedges
Change in accumulated other comprehensive income (loss), pretax
Other comprehensive income (loss), pretax		(10)	(10)	(12)	(14)
Change in accumulated other comprehensive income (loss), tax effect
Other comprehensive income (loss), tax effect		1	(2)	4	(1)
Change in accumulated other comprehensive income (loss), after-tax
Total other comprehensive income		$ (9)	$ (12)	$ (8)	$ (15)

[1] See Note 1 to the Consolidated Financial Statements in Citi’s 2023 Form 10-K.