CHANGES IN ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS) (AOCI) - Change in Each Component of AOCI (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)
Balance, beginning of period				$ 206,251
Balance, end of period		$ 209,144	$ 209,422	209,144	$ 209,422
Liability for future policyholder benefits		474	560	474	560
Accumulated other comprehensive income (loss)
Change in accumulated other comprehensive income (loss)
Balance, beginning of period		(45,729)	(45,441)	(44,800)	(47,062)
Other comprehensive income before reclassifications		(1,176)	(812)	(2,338)	420
Increase (decrease) due to amounts reclassified from AOCI		228	388	461	750
Total other comprehensive income		(948)	(424)	(1,877)	1,170
Balance, end of period		(46,677)	(45,865)	(46,677)	(45,865)
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		(45,729)	(45,441)	(44,800)	(47,035)
Net unrealized gains (losses) on debt securities
Change in accumulated other comprehensive income (loss)
Balance, beginning of period		(3,644)	(5,162)	(3,744)	(5,998)
Other comprehensive income before reclassifications		(24)	133	152	988
Increase (decrease) due to amounts reclassified from AOCI		(14)	(7)	(90)	(26)
Total other comprehensive income		(38)	126	62	962
Balance, end of period		(3,682)	(5,036)	(3,682)	(5,036)
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		(1,272)	517	(709)	842
Other comprehensive income before reclassifications		254	(613)	(319)	(940)
Increase (decrease) due to amounts reclassified from AOCI		2	(6)	12	(4)
Total other comprehensive income		256	(619)	(307)	(944)
Balance, end of period		(1,016)	(102)	(1,016)	(102)
Cash flow hedges
Change in accumulated other comprehensive income (loss)
Balance, beginning of period		(914)	(2,161)	(1,406)	(2,522)
Other comprehensive income before reclassifications		87	(206)	319	(200)
Increase (decrease) due to amounts reclassified from AOCI		198	377	458	732
Total other comprehensive income		285	171	777	532
Balance, end of period		(629)	(1,990)	(629)	(1,990)
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		(5,973)	(5,859)	(6,050)	(5,755)
Other comprehensive income before reclassifications		135	(170)	163	(302)
Increase (decrease) due to amounts reclassified from AOCI		44	34	93	62
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)
Balance, beginning of period					(5,755)
CTA, net of hedges
Change in accumulated other comprehensive income (loss)
Balance, beginning of period		(33,939)	(32,796)	(32,885)	(33,637)
Other comprehensive income before reclassifications		(1,634)	23	(2,688)	864
Total other comprehensive income		(1,634)	23	(2,688)	864
Balance, end of period		(35,573)	(32,773)	(35,573)	(32,773)
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		(42)	(12)	(40)	8
Other comprehensive income before reclassifications		4	27	12	11
Increase (decrease) due to amounts reclassified from AOCI		(1)	(10)	(11)	(14)
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)
Balance, beginning of period					8
Long-duration insurance contracts
Change in accumulated other comprehensive income (loss)
Balance, beginning of period		55	32	34	0
Other comprehensive income before reclassifications		2	(6)	23	(1)
Increase (decrease) due to amounts reclassified from AOCI		(1)		(1)
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)
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
Debt valuation adjustment (DVA) | Cumulative Effect, Period of Adoption, Adjusted Balance
Change in accumulated other comprehensive income (loss)
Balance, beginning of period					$ 842

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