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, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period				$ 206,251
Balance, end of period		$ 209,144	$ 209,422	209,144	$ 209,422
Citigroup's accumulated other comprehensive income (loss)
Change in accumulated other comprehensive income (loss), pretax
Balance, beginning of period, pretax		(53,391)	(53,443)	(52,422)	(55,253)
Other comprehensive income (loss), pretax		(711)	(521)	(1,680)	1,250
Balance, end of period, pretax		(54,102)	(53,964)	(54,102)	(53,964)
Change in accumulated other comprehensive income (loss), tax effect
Balance, beginning of period, tax effect		7,662	8,002	7,622	8,191
Other comprehensive income (loss), tax effect		(237)	97	(197)	(80)
Balance, end of period, tax effect		7,425	8,099	7,425	8,099
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period		(45,729)	(45,441)	(44,800)	(47,062)
Total other comprehensive income		(948)	(424)	(1,877)	1,170
Balance, end of period		(46,677)	(45,865)	(46,677)	(45,865)
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		(45,729)	(45,441)	(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		(52)	210	72	1,323
Change in accumulated other comprehensive income (loss), tax effect
Other comprehensive income (loss), tax effect		14	(84)	(10)	(361)
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period		(3,644)	(5,162)	(3,744)	(5,998)
Total other comprehensive income		(38)	126	62	962
Balance, end of period		(3,682)	(5,036)	(3,682)	(5,036)
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		343	(837)	(407)	(1,270)
Change in accumulated other comprehensive income (loss), tax effect
Other comprehensive income (loss), tax effect		(87)	218	100	326
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period		(1,272)	517	(709)	842
Total other comprehensive income		256	(619)	(307)	(944)
Balance, end of period		(1,016)	(102)	(1,016)	(102)
Excluded component of fair value hedges
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period		(42)	(12)	(40)	8
Total other comprehensive income		3	17	1	(3)
Balance, end of period		(39)	5	(39)	5
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		250	(156)	318	(312)
Change in accumulated other comprehensive income (loss), tax effect
Other comprehensive income (loss), tax effect		(71)	20	(62)	72
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period		(5,973)	(5,859)	(6,050)	(5,755)
Total other comprehensive income		179	(136)	256	(240)
Balance, end of period		(5,794)	(5,995)	(5,794)	(5,995)
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		(1,622)	15	(2,711)	803
Change in accumulated other comprehensive income (loss), tax effect
Other comprehensive income (loss), tax effect		(12)	8	23	61
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period		(33,939)	(32,796)	(32,885)	(33,637)
Total other comprehensive income		(1,634)	23	(2,688)	864
Balance, end of period		(35,573)	(32,773)	(35,573)	(32,773)
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		4	(8)	36	(2)
Change in accumulated other comprehensive income (loss), tax effect
Other comprehensive income (loss), tax effect		(3)	2	(14)	1
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period		55	32	34	0
Total other comprehensive income		1	(6)	22	(1)
Balance, end of period		56	26	56	26
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		364	233	1,014	712
Change in accumulated other comprehensive income (loss), tax effect
Other comprehensive income (loss), tax effect		(79)	(62)	(237)	(180)
Change in accumulated other comprehensive income (loss), after-tax
Total other comprehensive income		285	171	777	532
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		2	22	(2)	(4)
Change in accumulated other comprehensive income (loss), tax effect
Other comprehensive income (loss), tax effect		1	(5)	3	1
Change in accumulated other comprehensive income (loss), after-tax
Total other comprehensive income		$ 3	$ 17	$ 1	$ (3)

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