CHANGES IN ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS) (AOCI) - Schedule of Pre-Tax and After-Tax (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period	$ 193,946
Balance, end of period	192,982	$ 197,015
Citigroup's accumulated other comprehensive income (loss)
Change in accumulated other comprehensive income (loss), pretax
Balance, beginning of period, pretax	(42,772)	(44,082)
Other comprehensive income (loss), pretax	6,353	1,178
Balance, end of period, pretax	(36,419)	(42,904)
Change in accumulated other comprehensive income (loss), tax effect
Balance, beginning of period, tax effect	6,454	6,912
Other comprehensive income (loss), tax effect	(2,556)	(316)
Balance, end of period, tax effect	3,898	6,596
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period	(36,318)	(37,170)
Total other comprehensive income	3,797	862
Balance, end of period	(32,521)	(36,308)
Net unrealized gains (losses) on debt securities
Change in accumulated other comprehensive income (loss), pretax
Other comprehensive income (loss), pretax	4,121	1,500
Change in accumulated other comprehensive income (loss), tax effect
Other comprehensive income (loss), tax effect	(993)	(365)
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period	(265)	(2,250)
Total other comprehensive income	3,128	1,135
Balance, end of period	2,863	(1,115)
Debt valuation adjustment (DVA)
Change in accumulated other comprehensive income (loss), pretax
Other comprehensive income (loss), pretax	4,188	(725)
Change in accumulated other comprehensive income (loss), tax effect
Other comprehensive income (loss), tax effect	(1,048)	154
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period	(944)	192
Total other comprehensive income	3,140	(571)
Balance, end of period	2,196	(379)
Hedges
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period	(32)	(57)
Total other comprehensive income	27	18
Balance, end of period	(5)	(39)
Benefit plans
Change in accumulated other comprehensive income (loss), pretax
Other comprehensive income (loss), pretax	(418)	(68)
Change in accumulated other comprehensive income (loss), tax effect
Other comprehensive income (loss), tax effect	132	4
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period	(6,809)	(6,257)
Total other comprehensive income	(286)	(64)
Balance, end of period	(7,095)	(6,321)
Foreign currency translation adjustment
Change in accumulated other comprehensive income (loss), pretax
Other comprehensive income (loss), pretax	(4,055)	69
Change in accumulated other comprehensive income (loss), tax effect
Other comprehensive income (loss), tax effect	(54)	(11)
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period	(28,391)	(28,070)
Total other comprehensive income	(4,109)	58
Balance, end of period	(32,500)	(28,012)
Cash flow hedges | Hedges
Change in accumulated other comprehensive income (loss), pretax
Other comprehensive income (loss), pretax	2,484	378
Change in accumulated other comprehensive income (loss), tax effect
Other comprehensive income (loss), tax effect	(587)	(92)
Change in accumulated other comprehensive income (loss), after-tax
Total other comprehensive income	1,897	286
Fair value hedges | Hedges
Change in accumulated other comprehensive income (loss), pretax
Other comprehensive income (loss), pretax	33	24
Change in accumulated other comprehensive income (loss), tax effect
Other comprehensive income (loss), tax effect	(6)	(6)
Change in accumulated other comprehensive income (loss), after-tax
Total other comprehensive income	$ 27	$ 18