CHANGES IN ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS) (AOCI) - Schedule of Pre-Tax and After-Tax (Details) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2023	Mar. 31, 2022
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period		$ 201,838
Balance, end of period		209,019	$ 198,353
Change in net unrealized gains (losses) on debt securities
Change in accumulated other comprehensive income (loss), pretax
Other comprehensive income (loss), pretax		1,113	(5,624)
Change in accumulated other comprehensive income (loss), tax effect
Other comprehensive income (loss), tax effect		(277)	1,347
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period		(5,998)	(614)
Total other comprehensive income		836	(4,277)
Balance, end of period		(5,162)	(4,891)
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		(433)	1,050
Change in accumulated other comprehensive income (loss), tax effect
Other comprehensive income (loss), tax effect		108	(257)
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period		842	(1,187)
Total other comprehensive income		(325)	793
Balance, end of period		517	(394)
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		8	(47)
Total other comprehensive income		(20)	48
Balance, end of period		(12)	1
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)	177
Change in accumulated other comprehensive income (loss), tax effect
Other comprehensive income (loss), tax effect		52	(6)
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period		(5,755)	(5,852)
Total other comprehensive income		(104)	171
Balance, end of period		(5,859)	(5,681)
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		788	(69)
Change in accumulated other comprehensive income (loss), tax effect
Other comprehensive income (loss), tax effect		53	55
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period		(33,637)	(31,166)
Total other comprehensive income		841	(14)
Balance, end of period		(32,796)	(31,180)
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		6
Change in accumulated other comprehensive income (loss), tax effect
Other comprehensive income (loss), tax effect		(1)
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period		0	0
Total other comprehensive income		5	0
Balance, end of period		32	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		(55,253)	(45,383)
Other comprehensive income (loss), pretax		1,771	(6,424)
Balance, end of period, pretax		(53,443)	(51,807)
Change in accumulated other comprehensive income (loss), tax effect
Balance, beginning of period, tax effect		8,191	6,618
Other comprehensive income (loss), tax effect		(177)	1,604
Balance, end of period, tax effect		8,002	8,222
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period		(47,062)	(38,765)
Total other comprehensive income		1,594	(4,820)
Balance, end of period		(45,441)	(43,585)
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	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		(47,035)	(38,765)
Cash flow hedges | Hedges
Change in accumulated other comprehensive income (loss), pretax
Other comprehensive income (loss), pretax		479	(2,022)
Change in accumulated other comprehensive income (loss), tax effect
Other comprehensive income (loss), tax effect		(118)	481
Change in accumulated other comprehensive income (loss), after-tax
Total other comprehensive income		361	(1,541)
Fair value hedges | Hedges
Change in accumulated other comprehensive income (loss), pretax
Other comprehensive income (loss), pretax		(26)	64
Change in accumulated other comprehensive income (loss), tax effect
Other comprehensive income (loss), tax effect		6	(16)
Change in accumulated other comprehensive income (loss), after-tax
Total other comprehensive income		$ (20)	48
Fair value hedges | Long-duration insurance contracts
Change in accumulated other comprehensive income (loss), pretax
Other comprehensive income (loss), pretax			0
Change in accumulated other comprehensive income (loss), tax effect
Other comprehensive income (loss), tax effect			$ 0

[1] See Note 1 for additional details.