CHANGES IN ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS) (AOCI) - Schedule of Pre-Tax and After-Tax (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period	$ 200,200
Balance, end of period	203,273	$ 193,326
Change in net unrealized gains (losses) on debt securities
Change in accumulated other comprehensive income (loss), pretax
Other comprehensive income (loss), pretax	(2,427)	4,121
Change in accumulated other comprehensive income (loss), tax effect
Other comprehensive income (loss), tax effect	642	(993)
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period	3,320	(265)
Total other comprehensive income	(1,785)	3,128
Balance, end of period	1,535	2,863
Debt valuation adjustment (DVA)
Change in accumulated other comprehensive income (loss), pretax
Other comprehensive income (loss), pretax	(38)	4,188
Change in accumulated other comprehensive income (loss), tax effect
Other comprehensive income (loss), tax effect	(4)	(1,048)
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period	(1,419)	(944)
Total other comprehensive income	(42)	3,140
Balance, end of period	(1,461)	2,196
Hedges
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period	(47)	(32)
Total other comprehensive income	(10)	27
Balance, end of period	(57)	(5)
Benefit plans
Change in accumulated other comprehensive income (loss), pretax
Other comprehensive income (loss), pretax	907	(418)
Change in accumulated other comprehensive income (loss), tax effect
Other comprehensive income (loss), tax effect	(193)	132
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period	(6,864)	(6,809)
Total other comprehensive income	714	(286)
Balance, end of period	(6,150)	(7,095)
Foreign currency translation adjustment
Change in accumulated other comprehensive income (loss), pretax
Other comprehensive income (loss), pretax	(1,339)	(4,055)
Change in accumulated other comprehensive income (loss), tax effect
Other comprehensive income (loss), tax effect	65	(54)
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period	(28,641)	(28,391)
Total other comprehensive income	(1,274)	(4,109)
Balance, end of period	(29,915)	(32,500)
Citigroup's accumulated other comprehensive income (loss)
Change in accumulated other comprehensive income (loss), pretax
Balance, beginning of period, pretax	(36,992)	(42,772)
Other comprehensive income (loss), pretax	(3,639)	6,353
Balance, end of period, pretax	(40,631)	(36,419)
Change in accumulated other comprehensive income (loss), tax effect
Balance, beginning of period, tax effect	4,934	6,454
Other comprehensive income (loss), tax effect	686	(2,556)
Balance, end of period, tax effect	5,620	3,898
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period	(32,058)	(36,318)
Total other comprehensive income	(2,953)	3,797
Balance, end of period	(35,011)	(32,521)
Cash flow hedges | Hedges
Change in accumulated other comprehensive income (loss), pretax
Other comprehensive income (loss), pretax	(729)	2,484
Change in accumulated other comprehensive income (loss), tax effect
Other comprehensive income (loss), tax effect	173	(587)
Change in accumulated other comprehensive income (loss), after-tax
Total other comprehensive income	(556)	1,897
Fair value hedges | Hedges
Change in accumulated other comprehensive income (loss), pretax
Other comprehensive income (loss), pretax	(13)	33
Change in accumulated other comprehensive income (loss), tax effect
Other comprehensive income (loss), tax effect	3	(6)
Change in accumulated other comprehensive income (loss), after-tax
Total other comprehensive income	$ (10)	$ 27