CHANGES IN ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS) (AOCI) - Schedule of Pre-Tax and After-Tax (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), pretax
Pretax		4,799	2,633	(1,435)
Change in accumulated other comprehensive income (loss), tax effect
Tax effect		(1,214)	(648)	346
Change in accumulated other comprehensive income (loss)
Balance, beginning of year		(265)	(2,250)	(1,158)
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), pretax
Pretax		(616)	(1,473)	1,415
Change in accumulated other comprehensive income (loss), tax effect
Tax effect		141	337	(302)
Change in accumulated other comprehensive income (loss)
Balance, beginning of year		(944)	192	(921)
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)
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), pretax
Pretax		(78)	(671)	(94)
Change in accumulated other comprehensive income (loss), tax effect
Tax effect		23	119	20
Change in accumulated other comprehensive income (loss)
Balance, beginning of year		(6,809)	(6,257)	(6,183)
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
Change in accumulated other comprehensive income (loss), pretax
Pretax		(227)	(332)	(2,624)
Change in accumulated other comprehensive income (loss), tax effect
Tax effect		(23)	11	262
Change in accumulated other comprehensive income (loss)
Balance, beginning of year		(28,391)	(28,070)	(25,708)
Change, net of taxes		(250)	(321)	(2,362)
Balance, end of year		(28,641)	(28,391)	(28,070)
Foreign currency translation adjustment | Cumulative Effect, Period of Adoption, Adjustment
Change in accumulated other comprehensive income (loss)
Balance, beginning of year				0
Foreign currency translation adjustment | 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
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), pretax
AOCI, beginning balance, pretax		(42,772)	(44,082)	(41,228)
Pretax		5,780	1,310	(2,850)
AOCI, ending balance, pretax		(36,992)	(42,772)	(44,082)
Change in accumulated other comprehensive income (loss), tax effect
Balance, beginning of period, tax effect		6,454	6,912	6,560
Tax effect		(1,520)	(458)	351
Balance, end of period, tax effect		4,934	6,454	6,912
Change in accumulated other comprehensive income (loss)
Balance, beginning of year		(36,318)	(37,170)	(34,668)
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), pretax
AOCI, beginning balance, pretax				(4)
Change in accumulated other comprehensive income (loss), tax effect
Balance, beginning of period, tax effect				1
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), pretax
AOCI, beginning balance, pretax				(41,232)
Change in accumulated other comprehensive income (loss), tax effect
Balance, beginning of period, tax effect				6,561
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)
Cash flow hedges | Cash flow hedges
Change in accumulated other comprehensive income (loss), pretax
Pretax		1,925	1,120	(38)
Change in accumulated other comprehensive income (loss), tax effect
Tax effect		(455)	(269)	8
Change in accumulated other comprehensive income (loss)
Change, net of taxes		1,470	851	(30)
Fair value hedges | Excluded component of fair value hedges
Change in accumulated other comprehensive income (loss), pretax
Pretax		(23)	33	(74)
Change in accumulated other comprehensive income (loss), tax effect
Tax effect		8	(8)	17
Change in accumulated other comprehensive income (loss)
Change, net of taxes		$ (15)	$ 25	$ (57)

[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.