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, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period			$ 200,200
Balance, end of period	$ 201,610	$ 194,595	201,610	$ 194,595
Change in net unrealized gains (losses) on debt securities
Change in accumulated other comprehensive income (loss), pretax
Other comprehensive income (loss), pretax	(374)	(393)	(3,439)	4,905
Change in accumulated other comprehensive income (loss), tax effect
Other comprehensive income (loss), tax effect	95	111	901	(1,222)
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period	1,061	3,700	3,320	(265)
Total other comprehensive income	(279)	(282)	(2,538)	3,683
Balance, end of period	782	3,418	782	3,418
Debt valuation adjustment (DVA)
Change in accumulated other comprehensive income (loss), pretax
Other comprehensive income (loss), pretax	(107)	(452)	(256)	801
Change in accumulated other comprehensive income (loss), tax effect
Other comprehensive income (loss), tax effect	25	145	70	(200)
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period	(1,523)	(36)	(1,419)	(944)
Total other comprehensive income	(82)	(307)	(186)	601
Balance, end of period	(1,605)	(343)	(1,605)	(343)
Hedges
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period	(67)	8	(47)	(32)
Total other comprehensive income	8	(39)	(12)	1
Balance, end of period	(59)	(31)	(59)	(31)
Benefit plans
Change in accumulated other comprehensive income (loss), pretax
Other comprehensive income (loss), pretax	175	344	1,166	(166)
Change in accumulated other comprehensive income (loss), tax effect
Other comprehensive income (loss), tax effect	(40)	(98)	(230)	49
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period	(6,063)	(7,172)	(6,864)	(6,809)
Total other comprehensive income	135	246	936	(117)
Balance, end of period	(5,928)	(6,926)	(5,928)	(6,926)
Foreign currency translation adjustment
Change in accumulated other comprehensive income (loss), pretax
Other comprehensive income (loss), pretax	(1,325)	918	(2,217)	(2,652)
Change in accumulated other comprehensive income (loss), tax effect
Other comprehensive income (loss), tax effect	13	(21)	154	1
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period	(29,392)	(31,939)	(28,641)	(28,391)
Total other comprehensive income	(1,312)	897	(2,063)	(2,651)
Balance, end of period	(30,704)	(31,042)	(30,704)	(31,042)
Citigroup's accumulated other comprehensive income (loss)
Change in accumulated other comprehensive income (loss), pretax
Balance, beginning of period, pretax	(41,087)	(37,678)	(36,992)	(42,772)
Other comprehensive income (loss), pretax	(1,884)	59	(5,979)	5,153
Balance, end of period, pretax	(42,971)	(37,619)	(42,971)	(37,619)
Change in accumulated other comprehensive income (loss), tax effect
Balance, beginning of period, tax effect	5,967	4,333	4,934	6,454
Other comprehensive income (loss), tax effect	153	221	1,186	(1,900)
Balance, end of period, tax effect	6,120	4,554	6,120	4,554
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period	(35,120)	(33,345)	(32,058)	(36,318)
Total other comprehensive income	(1,731)	280	(4,793)	3,253
Balance, end of period	(36,851)	(33,065)	(36,851)	(33,065)
Cash flow hedges | Hedges
Change in accumulated other comprehensive income (loss), pretax
Other comprehensive income (loss), pretax	(265)	(307)	(1,219)	2,267
Change in accumulated other comprehensive income (loss), tax effect
Other comprehensive income (loss), tax effect	64	72	289	(531)
Change in accumulated other comprehensive income (loss), after-tax
Total other comprehensive income	(201)	(235)	(930)	1,736
Fair value hedges | Hedges
Change in accumulated other comprehensive income (loss), pretax
Other comprehensive income (loss), pretax	12	(51)	(14)	(2)
Change in accumulated other comprehensive income (loss), tax effect
Other comprehensive income (loss), tax effect	(4)	12	2	3
Change in accumulated other comprehensive income (loss), after-tax
Total other comprehensive income	$ 8	$ (39)	$ (12)	$ 1