CHANGES IN ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS) (AOCI) - Change in Each Component of AOCI (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)
Balance, beginning of period				$ 206,251
Balance, end of period		$ 209,902	$ 210,195	209,902	$ 210,195
Liability for future policyholder benefits		463	519	463	519
Accumulated other comprehensive income (loss)
Change in accumulated other comprehensive income (loss)
Balance, beginning of period		(46,677)	(45,865)	(44,800)	(47,062)
Other comprehensive income before reclassifications		1,313	(722)	(1,025)	(302)
Increase (decrease) due to amounts reclassified from AOCI		167	410	628	1,160
Total other comprehensive income		1,480	(312)	(397)	858
Balance, end of period		(45,197)	(46,177)	(45,197)	(46,177)
Accumulated other comprehensive income (loss) | Cumulative Effect, Period of Adoption, Adjustment
Change in accumulated other comprehensive income (loss)
Balance, beginning of period	[1]				27
Accumulated other comprehensive income (loss) | Cumulative Effect, Period of Adoption, Adjusted Balance
Change in accumulated other comprehensive income (loss)
Balance, beginning of period		(46,677)	(45,865)	(44,800)	(47,035)
Net unrealized gains (losses) on debt securities
Change in accumulated other comprehensive income (loss)
Balance, beginning of period		(3,682)	(5,036)	(3,744)	(5,998)
Other comprehensive income before reclassifications		1,381	(176)	1,533	812
Increase (decrease) due to amounts reclassified from AOCI		(46)	7	(136)	(19)
Total other comprehensive income		1,335	(169)	1,397	793
Balance, end of period		(2,347)	(5,205)	(2,347)	(5,205)
Net unrealized gains (losses) on debt securities | Cumulative Effect, Period of Adoption, Adjusted Balance
Change in accumulated other comprehensive income (loss)
Balance, beginning of period					(5,998)
Debt valuation adjustment (DVA)
Change in accumulated other comprehensive income (loss)
Balance, beginning of period		(1,016)	(102)	(709)	842
Other comprehensive income before reclassifications		(155)	290	(474)	(650)
Increase (decrease) due to amounts reclassified from AOCI		5	9	17	5
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)
Balance, beginning of period					842
Cash flow hedges
Change in accumulated other comprehensive income (loss)
Balance, beginning of period		(629)	(1,990)	(1,406)	(2,522)
Other comprehensive income before reclassifications		(305)	366	14	166
Increase (decrease) due to amounts reclassified from AOCI		161	365	619	1,097
Total other comprehensive income		(144)	731	633	1,263
Balance, end of period		(773)	(1,259)	(773)	(1,259)
Cash flow hedges | Cumulative Effect, Period of Adoption, Adjusted Balance
Change in accumulated other comprehensive income (loss)
Balance, beginning of period					(2,522)
Benefit plans
Change in accumulated other comprehensive income (loss)
Balance, beginning of period		(5,794)	(5,995)	(6,050)	(5,755)
Other comprehensive income before reclassifications		1	274	164	(28)
Increase (decrease) due to amounts reclassified from AOCI		48	38	141	100
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)
Balance, beginning of period					(5,755)
CTA, net of hedges
Change in accumulated other comprehensive income (loss)
Balance, beginning of period		(35,573)	(32,773)	(32,885)	(33,637)
Other comprehensive income before reclassifications		416	(1,496)	(2,272)	(632)
Total other comprehensive income		416	(1,496)	(2,272)	(632)
Balance, end of period		(35,157)	(34,269)	(35,157)	(34,269)
CTA, net of hedges | Cumulative Effect, Period of Adoption, Adjusted Balance
Change in accumulated other comprehensive income (loss)
Balance, beginning of period					(33,637)
Excluded component of fair value hedges
Change in accumulated other comprehensive income (loss)
Balance, beginning of period		(39)	5	(40)	8
Other comprehensive income before reclassifications		(8)	(3)	4	8
Increase (decrease) due to amounts reclassified from AOCI		(1)	(9)	(12)	(23)
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)
Balance, beginning of period					8
Long-duration insurance contracts
Change in accumulated other comprehensive income (loss)
Balance, beginning of period		56	26	34	0
Other comprehensive income before reclassifications		(17)	23	6	22
Increase (decrease) due to amounts reclassified from AOCI		0		(1)
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)
Balance, beginning of period					27
Long-duration insurance contracts | Cumulative Effect, Period of Adoption, Adjusted Balance
Change in accumulated other comprehensive income (loss)
Balance, beginning of period					$ 27

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