CHANGES IN ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS) (AOCI) - Schedule of Pre-Tax and After-Tax (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period	$ 202,672
Balance, end of period	198,353	$ 203,273
Change in net unrealized gains (losses) on debt securities
Change in accumulated other comprehensive income (loss), pretax
Other comprehensive income (loss), pretax	(5,624)	(2,427)
Change in accumulated other comprehensive income (loss), tax effect
Other comprehensive income (loss), tax effect	1,347	642
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period	(614)	3,320
Total other comprehensive income	(4,277)	(1,785)
Balance, end of period	(4,891)	1,535
Debt valuation adjustment (DVA)
Change in accumulated other comprehensive income (loss), pretax
Other comprehensive income (loss), pretax	1,050	(38)
Change in accumulated other comprehensive income (loss), tax effect
Other comprehensive income (loss), tax effect	(257)	(4)
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period	(1,187)	(1,419)
Total other comprehensive income	793	(42)
Balance, end of period	(394)	(1,461)
Hedges
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period	(47)	(47)
Total other comprehensive income	48	(10)
Balance, end of period	1	(57)
Benefit plans
Change in accumulated other comprehensive income (loss), pretax
Other comprehensive income (loss), pretax	177	907
Change in accumulated other comprehensive income (loss), tax effect
Other comprehensive income (loss), tax effect	(6)	(193)
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period	(5,852)	(6,864)
Total other comprehensive income	171	714
Balance, end of period	(5,681)	(6,150)
Foreign currency translation adjustment
Change in accumulated other comprehensive income (loss), pretax
Other comprehensive income (loss), pretax	(69)	(1,339)
Change in accumulated other comprehensive income (loss), tax effect
Other comprehensive income (loss), tax effect	55	65
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period	(31,166)	(28,641)
Total other comprehensive income	(14)	(1,274)
Balance, end of period	(31,180)	(29,915)
Citigroup's accumulated other comprehensive income (loss)
Change in accumulated other comprehensive income (loss), pretax
Balance, beginning of period, pretax	(45,383)	(36,992)
Other comprehensive income (loss), pretax	(6,424)	(3,639)
Balance, end of period, pretax	(51,807)	(40,631)
Change in accumulated other comprehensive income (loss), tax effect
Balance, beginning of period, tax effect	6,618	4,934
Other comprehensive income (loss), tax effect	1,604	686
Balance, end of period, tax effect	8,222	5,620
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period	(38,765)	(32,058)
Total other comprehensive income	(4,820)	(2,953)
Balance, end of period	(43,585)	(35,011)
Cash flow hedges | Hedges
Change in accumulated other comprehensive income (loss), pretax
Other comprehensive income (loss), pretax	(2,022)	(729)
Change in accumulated other comprehensive income (loss), tax effect
Other comprehensive income (loss), tax effect	481	173
Change in accumulated other comprehensive income (loss), after-tax
Total other comprehensive income	(1,541)	(556)
Fair value hedges | Hedges
Change in accumulated other comprehensive income (loss), pretax
Other comprehensive income (loss), pretax	64	(13)
Change in accumulated other comprehensive income (loss), tax effect
Other comprehensive income (loss), tax effect	(16)	3
Change in accumulated other comprehensive income (loss), after-tax
Total other comprehensive income	$ 48	$ (10)