CHANGES IN ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS) (AOCI) - Schedule of Pre-Tax and After-Tax (Details) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2024	Mar. 31, 2023
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period		$ 206,251
Balance, end of period		207,398	$ 209,019
Change in net unrealized gains (losses) on debt securities
Change in accumulated other comprehensive income (loss), pretax
Other comprehensive income (loss), pretax		124	1,113
Change in accumulated other comprehensive income (loss), tax effect
Other comprehensive income (loss), tax effect		(24)	(277)
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period		(3,744)	(5,998)
Total other comprehensive income		100	836
Balance, end of period		(3,644)	(5,162)
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		(750)	(433)
Change in accumulated other comprehensive income (loss), tax effect
Other comprehensive income (loss), tax effect		187	108
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period		(709)	842
Total other comprehensive income		(563)	(325)
Balance, end of period		(1,272)	517
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		(40)	8
Total other comprehensive income		(2)	(20)
Balance, end of period		(42)	(12)
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		68	(156)
Change in accumulated other comprehensive income (loss), tax effect
Other comprehensive income (loss), tax effect		9	52
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period		(6,050)	(5,755)
Total other comprehensive income		77	(104)
Balance, end of period		(5,973)	(5,859)
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 (CTA)
Change in accumulated other comprehensive income (loss), pretax
Other comprehensive income (loss), pretax		(1,089)	788
Change in accumulated other comprehensive income (loss), tax effect
Other comprehensive income (loss), tax effect		35	53
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period		(32,885)	(33,637)
Total other comprehensive income		(1,054)	841
Balance, end of period		(33,939)	(32,796)
Foreign currency translation adjustment (CTA) | Cumulative Effect, Period of Adoption, Adjustment
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period			0
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		32
Change in accumulated other comprehensive income (loss), tax effect
Other comprehensive income (loss), tax effect		(11)
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period		34	0
Total other comprehensive income		21	5
Balance, end of period		55	32
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		(52,422)	(55,253)
Other comprehensive income (loss), pretax		(969)	1,771
Balance, end of period, pretax		(53,391)	(53,443)
Change in accumulated other comprehensive income (loss), tax effect
Balance, beginning of period, tax effect		7,622	8,191
Other comprehensive income (loss), tax effect		40	(177)
Balance, end of period, tax effect		7,662	8,002
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period		(44,800)	(47,062)
Total other comprehensive income		(929)	1,594
Balance, end of period		(45,729)	(45,441)
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]	0	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		(44,800)	(47,035)
Cash flow hedges | Hedges
Change in accumulated other comprehensive income (loss), pretax
Other comprehensive income (loss), pretax		650	479
Change in accumulated other comprehensive income (loss), tax effect
Other comprehensive income (loss), tax effect		(158)	(118)
Change in accumulated other comprehensive income (loss), after-tax
Total other comprehensive income		492	361
Excluded component of fair value hedges | Hedges
Change in accumulated other comprehensive income (loss), pretax
Other comprehensive income (loss), pretax		(4)	(26)
Change in accumulated other comprehensive income (loss), tax effect
Other comprehensive income (loss), tax effect		2	6
Change in accumulated other comprehensive income (loss), after-tax
Total other comprehensive income		$ (2)	(20)
Excluded component of fair value hedges | Long-duration insurance contracts
Change in accumulated other comprehensive income (loss), pretax
Other comprehensive income (loss), pretax			6
Change in accumulated other comprehensive income (loss), tax effect
Other comprehensive income (loss), tax effect			$ (1)

[1] See Note 1 for additional details.