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, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period			$ 200,200
Balance, end of period	$ 202,910	$ 192,386	202,910	$ 192,386
Change in net unrealized gains (losses) on debt securities
Change in accumulated other comprehensive income (loss), pretax
Other comprehensive income (loss), pretax	(638)	1,178	(3,065)	5,298
Change in accumulated other comprehensive income (loss), tax effect
Other comprehensive income (loss), tax effect	164	(341)	806	(1,333)
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period	1,535	2,863	3,320	(265)
Total other comprehensive income	(474)	837	(2,259)	3,965
Balance, end of period	1,061	3,700	1,061	3,700
Debt valuation adjustment (DVA)
Change in accumulated other comprehensive income (loss), pretax
Other comprehensive income (loss), pretax	(110)	(2,935)	(148)	1,253
Change in accumulated other comprehensive income (loss), tax effect
Other comprehensive income (loss), tax effect	48	703	44	(345)
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period	(1,461)	2,196	(1,419)	(944)
Total other comprehensive income	(62)	(2,232)	(104)	908
Balance, end of period	(1,523)	(36)	(1,523)	(36)
Hedges
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period	(57)	(5)	(47)	(32)
Total other comprehensive income	(10)	13	(20)	40
Balance, end of period	(67)	8	(67)	8
Benefit plans
Change in accumulated other comprehensive income (loss), pretax
Other comprehensive income (loss), pretax	84	(93)	991	(510)
Change in accumulated other comprehensive income (loss), tax effect
Other comprehensive income (loss), tax effect	3	16	(190)	147
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period	(6,150)	(7,095)	(6,864)	(6,809)
Total other comprehensive income	87	(77)	801	(363)
Balance, end of period	(6,063)	(7,172)	(6,063)	(7,172)
Foreign currency translation adjustment
Change in accumulated other comprehensive income (loss), pretax
Other comprehensive income (loss), pretax	445	485	(894)	(3,570)
Change in accumulated other comprehensive income (loss), tax effect
Other comprehensive income (loss), tax effect	78	76	143	22
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period	(29,915)	(32,500)	(28,641)	(28,391)
Total other comprehensive income	523	561	(751)	(3,548)
Balance, end of period	(29,392)	(31,939)	(29,392)	(31,939)
Citigroup's accumulated other comprehensive income (loss)
Change in accumulated other comprehensive income (loss), pretax
Balance, beginning of period, pretax	(40,631)	(36,419)	(36,992)	(42,772)
Other comprehensive income (loss), pretax	(456)	(1,259)	(4,095)	5,094
Balance, end of period, pretax	(41,087)	(37,678)	(41,087)	(37,678)
Change in accumulated other comprehensive income (loss), tax effect
Balance, beginning of period, tax effect	5,620	3,898	4,934	6,454
Other comprehensive income (loss), tax effect	347	435	1,033	(2,121)
Balance, end of period, tax effect	5,967	4,333	5,967	4,333
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period	(35,011)	(32,521)	(32,058)	(36,318)
Total other comprehensive income	(109)	(824)	(3,062)	2,973
Balance, end of period	(35,120)	(33,345)	(35,120)	(33,345)
Cash flow hedges | Hedges
Change in accumulated other comprehensive income (loss), pretax
Other comprehensive income (loss), pretax	(224)	90	(953)	2,574
Change in accumulated other comprehensive income (loss), tax effect
Other comprehensive income (loss), tax effect	51	(16)	224	(603)
Change in accumulated other comprehensive income (loss), after-tax
Total other comprehensive income	(173)	74	(729)	1,971
Fair value hedges | Hedges
Change in accumulated other comprehensive income (loss), pretax
Other comprehensive income (loss), pretax	(13)	16	(26)	49
Change in accumulated other comprehensive income (loss), tax effect
Other comprehensive income (loss), tax effect	3	(3)	6	(9)
Change in accumulated other comprehensive income (loss), after-tax
Total other comprehensive income	$ (10)	$ 13	$ (20)	$ 40