Cover Page - USD ($) $ in Billions	12 Months Ended
	Dec. 31, 2020	Jan. 31, 2021	Jun. 30, 2020
Entity Information [Line Items]
Document Type	10-K
Document Annual Report	true
Document Period End Date	Dec. 31, 2020
Current Fiscal Year End Date	--12-31
Document Transition Report	false
Entity File Number	1-9924
Entity Registrant Name	Citigroup Inc
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	52-1568099
Entity Address, Address Line One	388 Greenwich Street,
Entity Address, City or Town	New York
Entity Address, State or Province	NY
Entity Address, Postal Zip Code	10013
City Area Code	212
Local Phone Number	559-1000
Entity Well-known Seasoned Issuer	No
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
ICFR Auditor Attestation Flag	true
Entity Shell Company	false
Entity Public Float			$ 106.2
Entity Common Stock, Shares Outstanding		2,087,317,952
Documents Incorporated by Reference	Portions of the registrant’s proxy statement for the annual meeting of stockholders scheduled to be held on April 27, 2021 are incorporated by reference in this Form 10-K in response to Items 10, 11, 12, 13 and 14 of Part III.
Entity Central Index Key	0000831001
Amendment Flag	false
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	FY
Common Stock, par value $.01 per share
Entity Information [Line Items]
Title of 12(b) Security	Common Stock, par value $.01 per share
Trading Symbol	C
Security Exchange Name	NYSE
Depositary Shares, each representing 1/1,000th interest in a share of 7.125% Fixed/Floating Rate Noncumulative Preferred Stock, Series J
Entity Information [Line Items]
Title of 12(b) Security	Dep Shs, represent 1/1,000th interest in a share of 7.125% Fix/Float Rate Noncum Pref Stk, Ser J
Trading Symbol	C Pr J
Security Exchange Name	NYSE
Depositary Shares, each representing 1/1,000th interest in a share of 6.875% Fixed/Floating Rate Noncumulative Preferred Stock, Series K
Entity Information [Line Items]
Title of 12(b) Security	Dep Shs, represent 1/1,000th interest in a share of 6.875% Fix/Float Rate Noncum Pref Stk, Ser K
Trading Symbol	C Pr K
Security Exchange Name	NYSE
7.625% Trust Preferred Securities of Citigroup Capital III (and registrant’s guaranty with respect thereto)
Entity Information [Line Items]
Title of 12(b) Security	7.625% TRUPs of Cap III (and registrant’s guaranty)
Trading Symbol	C/36Y
Security Exchange Name	NYSE
7.875% Fixed Rate / Floating Rate Trust Preferred Securities (TruPS®) of Citigroup Capital XIII (and registrant’s guaranty with respect thereto)
Entity Information [Line Items]
Title of 12(b) Security	7.875% FXD / FRN TruPS of Cap XIII (and registrant’s guaranty)
Trading Symbol	C N
Security Exchange Name	NYSE
6.829% Fixed Rate / Floating Rate Enhanced Trust Preferred Securities (Enhanced TruPS®) of Citigroup Capital XVIII (and registrant’s guaranty with respect thereto)
Entity Information [Line Items]
Title of 12(b) Security	6.829% FXD / FRN Enhanced TruPS of Cap XVIII (and registrant’s guaranty)
Trading Symbol	C/67BP
Security Exchange Name	NYSE
Medium-Term Senior Notes, Series N, Callable Step-Up Coupon Notes Due March 31, 2036 of CGMHI (and registrant’s guaranty with respect thereto)
Entity Information [Line Items]
Title of 12(b) Security	MTN, Series N, Callable Step-Up Coupon Notes due Mar 2036 of CGMHI (and registrant’s guaranty)
Trading Symbol	C/36A
Security Exchange Name	NYSE
Medium-Term Senior Notes, Series N, Callable Fixed Rate Notes Due December 18, 2035 of CGMHI (and registrant’s guaranty with respect thereto)
Entity Information [Line Items]
Title of 12(b) Security	MTN, Series N, Callable Fixed Rate Notes Due Dec 2035 of CGMHI (and registrant’s guaranty)
Trading Symbol	C/35
Security Exchange Name	NYSE