CHANGES IN ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS) (AOCI) - Schedule of Pre-Tax and After-Tax (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period			$ 209,366
Balance, end of period	$ 213,877	$ 209,902	213,877	$ 209,902
Citigroup's accumulated other comprehensive income (loss)
Change in accumulated other comprehensive income (loss), pretax
Balance, beginning of period, pretax	(49,927)	(54,102)	(54,439)	(52,422)
Other comprehensive income (loss), pretax	(551)	2,099	3,961	419
Balance, end of period, pretax	(50,478)	(52,003)	(50,478)	(52,003)
Change in accumulated other comprehensive income (loss), tax effect
Balance, beginning of period, tax effect	6,141	7,425	6,587	7,622
Other comprehensive income (loss), tax effect	167	(619)	(279)	(816)
Balance, end of period, tax effect	6,308	6,806	6,308	6,806
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period	(43,786)	(46,677)	(47,852)	(44,800)
Total other comprehensive income	(384)	1,480	3,682	(397)
Balance, end of period	(44,170)	(45,197)	(44,170)	(45,197)
Citigroup's accumulated other comprehensive income (loss) | Adjusted balance, beginning of period
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period				(44,800)
Change in net unrealized gains (losses) on debt securities
Change in accumulated other comprehensive income (loss), pretax
Other comprehensive income (loss), pretax	613	1,781	1,720	1,853
Change in accumulated other comprehensive income (loss), tax effect
Other comprehensive income (loss), tax effect	(163)	(446)	(477)	(456)
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period	(2,044)	(3,682)	(2,837)	(3,744)
Total other comprehensive income	450	1,335	1,243	1,397
Balance, end of period	(1,594)	(2,347)	(1,594)	(2,347)
Debt valuation adjustment (DVA)
Change in accumulated other comprehensive income (loss), pretax
Other comprehensive income (loss), pretax	(1,293)	(201)	(684)	(608)
Change in accumulated other comprehensive income (loss), tax effect
Other comprehensive income (loss), tax effect	272	51	100	151
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period	(684)	(1,016)	(1,121)	(709)
Total other comprehensive income	(1,021)	(150)	(584)	(457)
Balance, end of period	(1,705)	(1,166)	(1,705)	(1,166)
Excluded component of fair value hedges
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period	(45)	(39)	(52)	(40)
Total other comprehensive income	11	(9)	18	(8)
Balance, end of period	(34)	(48)	(34)	(48)
Benefit plans
Change in accumulated other comprehensive income (loss), pretax
Other comprehensive income (loss), pretax	20	88	(55)	405
Change in accumulated other comprehensive income (loss), tax effect
Other comprehensive income (loss), tax effect	6	(39)	18	(100)
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period	(5,690)	(5,794)	(5,627)	(6,050)
Total other comprehensive income	26	49	(37)	305
Balance, end of period	(5,664)	(5,745)	(5,664)	(5,745)
Foreign currency translation adjustment (CTA)
Change in accumulated other comprehensive income (loss), pretax
Other comprehensive income (loss), pretax	67	638	2,834	(2,071)
Change in accumulated other comprehensive income (loss), tax effect
Other comprehensive income (loss), tax effect	68	(222)	116	(201)
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period	(35,232)	(35,573)	(38,047)	(32,885)
Total other comprehensive income	135	416	2,950	(2,272)
Balance, end of period	(35,097)	(35,157)	(35,097)	(35,157)
Long-duration insurance contracts
Change in accumulated other comprehensive income (loss), pretax
Other comprehensive income (loss), pretax	(16)	(26)	(16)	9
Change in accumulated other comprehensive income (loss), tax effect
Other comprehensive income (loss), tax effect	6	9	4	(4)
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period	50	56	52	34
Total other comprehensive income	(10)	(17)	(12)	5
Balance, end of period	40	39	40	39
Cash flow hedges | Excluded component of fair value hedges
Change in accumulated other comprehensive income (loss), pretax
Other comprehensive income (loss), pretax	44	(171)	140	843
Change in accumulated other comprehensive income (loss), tax effect
Other comprehensive income (loss), tax effect	(19)	27	(36)	(210)
Change in accumulated other comprehensive income (loss), after-tax
Total other comprehensive income	25	(144)	104	633
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	14	(10)	22	(12)
Change in accumulated other comprehensive income (loss), tax effect
Other comprehensive income (loss), tax effect	(3)	1	(4)	4
Change in accumulated other comprehensive income (loss), after-tax
Total other comprehensive income	$ 11	$ (9)	$ 18	$ (8)