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, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period			$ 193,946
Balance, end of period	$ 194,595	$ 197,070	194,595	$ 197,070
Change in net unrealized gains (losses) on debt securities
Change in accumulated other comprehensive income (loss), pretax
Other comprehensive income (loss), pretax	(393)	419	4,905	2,855
Change in accumulated other comprehensive income (loss), tax effect
Other comprehensive income (loss), tax effect	111	(112)	(1,222)	(710)
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period	3,700	(412)	(265)	(2,250)
Total other comprehensive income	(282)	307	3,683	2,145
Balance, end of period	3,418	(105)	3,418	(105)
Debt valuation adjustment (DVA)
Change in accumulated other comprehensive income (loss), pretax
Other comprehensive income (loss), pretax	(452)	273	801	(449)
Change in accumulated other comprehensive income (loss), tax effect
Other comprehensive income (loss), tax effect	145	(63)	(200)	91
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period	(36)	(376)	(944)	192
Total other comprehensive income	(307)	210	601	(358)
Balance, end of period	(343)	(166)	(343)	(166)
Hedges
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period	8	5	(32)	(57)
Total other comprehensive income	(39)	(10)	1	52
Balance, end of period	(31)	(5)	(31)	(5)
Benefit plans
Change in accumulated other comprehensive income (loss), pretax
Other comprehensive income (loss), pretax	344	(356)	(166)	(753)
Change in accumulated other comprehensive income (loss), tax effect
Other comprehensive income (loss), tax effect	(98)	106	49	186
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period	(7,172)	(6,574)	(6,809)	(6,257)
Total other comprehensive income	246	(250)	(117)	(567)
Balance, end of period	(6,926)	(6,824)	(6,926)	(6,824)
Foreign currency translation adjustment
Change in accumulated other comprehensive income (loss), pretax
Other comprehensive income (loss), pretax	918	(1,442)	(2,652)	(1,290)
Change in accumulated other comprehensive income (loss), tax effect
Other comprehensive income (loss), tax effect	(21)	0	1	(3)
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period	(31,939)	(27,921)	(28,391)	(28,070)
Total other comprehensive income	897	(1,442)	(2,651)	(1,293)
Balance, end of period	(31,042)	(29,363)	(31,042)	(29,363)
Citigroup's accumulated other comprehensive income (loss)
Change in accumulated other comprehensive income (loss), pretax
Balance, beginning of period, pretax	(37,678)	(41,472)	(42,772)	(44,082)
Other comprehensive income (loss), pretax	59	(783)	5,153	1,827
Balance, end of period, pretax	(37,619)	(42,255)	(37,619)	(42,255)
Change in accumulated other comprehensive income (loss), tax effect
Balance, beginning of period, tax effect	4,333	6,269	6,454	6,912
Other comprehensive income (loss), tax effect	221	(149)	(1,900)	(792)
Balance, end of period, tax effect	4,554	6,120	4,554	6,120
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period	(33,345)	(35,203)	(36,318)	(37,170)
Total other comprehensive income	280	(932)	3,253	1,035
Balance, end of period	(33,065)	(36,135)	(33,065)	(36,135)
Cash flow hedges | Hedges
Change in accumulated other comprehensive income (loss), pretax
Other comprehensive income (loss), pretax	(307)	333	2,267	1,391
Change in accumulated other comprehensive income (loss), tax effect
Other comprehensive income (loss), tax effect	72	(80)	(531)	(335)
Change in accumulated other comprehensive income (loss), after-tax
Total other comprehensive income	(235)	253	1,736	1,056
Fair value hedges | Hedges
Change in accumulated other comprehensive income (loss), pretax
Other comprehensive income (loss), pretax	(51)	(10)	(2)	73
Change in accumulated other comprehensive income (loss), tax effect
Other comprehensive income (loss), tax effect	12	0	3	(21)
Change in accumulated other comprehensive income (loss), after-tax
Total other comprehensive income	$ (39)	$ (10)	$ 1	$ 52