CHANGES IN ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS) (AOCI) - Change in Each Component of AOCI (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Change in accumulated other comprehensive income (loss)
Balance, beginning of year		$ 193,946	$ 197,074
Balance, end of year		200,200	193,946	$ 197,074
Net unrealized gains (losses) on investment securities
Change in accumulated other comprehensive income (loss)
Balance, beginning of year		(265)	(2,250)	(1,158)
Other comprehensive income before reclassifications		4,837	3,065	(866)
Increase (decrease) due to amounts reclassified from AOCI		(1,252)	(1,080)	(223)
Change, net of taxes		3,585	1,985	(1,089)
Balance, end of year		3,320	(265)	(2,250)
Net unrealized gains (losses) on investment securities | Cumulative Effect, Period of Adoption, Adjustment
Change in accumulated other comprehensive income (loss)
Balance, beginning of year				(3)
Net unrealized gains (losses) on investment securities | Cumulative Effect, Period of Adoption, Adjusted Balance
Change in accumulated other comprehensive income (loss)
Balance, beginning of year				(1,161)
Debt valuation adjustment (DVA)
Change in accumulated other comprehensive income (loss)
Balance, beginning of year		(944)	192	(921)
Other comprehensive income before reclassifications		(490)	(1,151)	1,081
Increase (decrease) due to amounts reclassified from AOCI		15	15	32
Change, net of taxes		(475)	(1,136)	1,113
Balance, end of year		(1,419)	(944)	192
Debt valuation adjustment (DVA) | Cumulative Effect, Period of Adoption, Adjustment
Change in accumulated other comprehensive income (loss)
Balance, beginning of year				0
Debt valuation adjustment (DVA) | Cumulative Effect, Period of Adoption, Adjusted Balance
Change in accumulated other comprehensive income (loss)
Balance, beginning of year				(921)
Cash flow hedges
Change in accumulated other comprehensive income (loss)
Balance, beginning of year		123	(728)	(698)
Other comprehensive income before reclassifications		2,027	549	(135)
Increase (decrease) due to amounts reclassified from AOCI		(557)	302	105
Change, net of taxes		1,470	851	(30)
Balance, end of year		1,593	123	(728)
Cash flow hedges | Cumulative Effect, Period of Adoption, Adjustment
Change in accumulated other comprehensive income (loss)
Balance, beginning of year				0
Cash flow hedges | Cumulative Effect, Period of Adoption, Adjusted Balance
Change in accumulated other comprehensive income (loss)
Balance, beginning of year				(698)
Benefit plans
Change in accumulated other comprehensive income (loss)
Balance, beginning of year		(6,809)	(6,257)	(6,183)
Other comprehensive income before reclassifications		(287)	(758)	(240)
Increase (decrease) due to amounts reclassified from AOCI		232	206	166
Change, net of taxes		(55)	(552)	(74)
Balance, end of year		(6,864)	(6,809)	(6,257)
Benefit plans | Cumulative Effect, Period of Adoption, Adjustment
Change in accumulated other comprehensive income (loss)
Balance, beginning of year				0
Benefit plans | Cumulative Effect, Period of Adoption, Adjusted Balance
Change in accumulated other comprehensive income (loss)
Balance, beginning of year				(6,183)
Foreign currency translation adjustment (CTA), net of hedges
Change in accumulated other comprehensive income (loss)
Balance, beginning of year		(28,391)	(28,070)	(25,708)
Other comprehensive income before reclassifications		(250)	(321)	(2,607)
Increase (decrease) due to amounts reclassified from AOCI		0	0	245
Change, net of taxes		(250)	(321)	(2,362)
Balance, end of year		(28,641)	(28,391)	(28,070)
Foreign currency translation adjustment (CTA), net of hedges | Cumulative Effect, Period of Adoption, Adjustment
Change in accumulated other comprehensive income (loss)
Balance, beginning of year				0
Foreign currency translation adjustment (CTA), net of hedges | Cumulative Effect, Period of Adoption, Adjusted Balance
Change in accumulated other comprehensive income (loss)
Balance, beginning of year				(25,708)
Excluded component of fair value hedges
Change in accumulated other comprehensive income (loss)
Balance, beginning of year		(32)	(57)	0
Other comprehensive income before reclassifications		(15)	25	(57)
Increase (decrease) due to amounts reclassified from AOCI		0	0	0
Change, net of taxes		(15)	25	(57)
Balance, end of year		(47)	(32)	(57)
Excluded component of fair value hedges | Cumulative Effect, Period of Adoption, Adjustment
Change in accumulated other comprehensive income (loss)
Balance, beginning of year				0
Excluded component of fair value hedges | Cumulative Effect, Period of Adoption, Adjusted Balance
Change in accumulated other comprehensive income (loss)
Balance, beginning of year				0
Accumulated other comprehensive income (loss)
Change in accumulated other comprehensive income (loss)
Balance, beginning of year		(36,318)	(37,170)	(34,668)
Other comprehensive income before reclassifications		5,822	1,409	(2,824)
Increase (decrease) due to amounts reclassified from AOCI		(1,562)	(557)	325
Change, net of taxes	[1]	4,260	852	(2,499)
Balance, end of year		(32,058)	(36,318)	(37,170)
Accumulated other comprehensive income (loss) | Cumulative Effect, Period of Adoption, Adjustment
Change in accumulated other comprehensive income (loss)
Balance, beginning of year	[2]			(3)
Accumulated other comprehensive income (loss) | Cumulative Effect, Period of Adoption, Adjusted Balance
Change in accumulated other comprehensive income (loss)
Balance, beginning of year		$ (36,318)	(37,170)	(34,671)
Balance, end of year			$ (36,318)	$ (37,170)

[1] Includes treasury stock related to (i) certain activity on employee stock option program exercises where the employee delivers existing shares to cover the option exercise, or (ii) under Citi’s employee restricted or deferred stock programs where shares are withheld to satisfy tax requirements.

[2] See Note 1 to the Consolidated Financial Statements for additional details.