SECURITIZATIONS AND VARIABLE INTEREST ENTITIES - Assets and Liabilities of VIEs (Details) - USD ($) $ in Millions	Sep. 30, 2023	Jun. 30, 2023	Dec. 31, 2022	Sep. 30, 2022	Jun. 30, 2022	Dec. 31, 2021
Assets of consolidated VIEs to be used to settle obligations of consolidated VIEs
Cash	$ 26,548		$ 30,577	$ 26,502
Trading account assets	406,368		334,114
Investments	508,998		526,582
Loans, net of unearned income	666,348		657,221
Allowance for credit losses on loans (ACLL)	(17,629)	$ (17,496)	(16,974)	(16,309)	$ (15,952)	$ (16,455)
Total loans, net	648,719		640,247
Other assets, at fair value	96,824		103,743
Total assets	2,368,477		2,416,676
Liabilities of consolidated VIEs for which creditors or beneficial interest holders do not have recourse to the general credit of Citigroup
Short-term borrowings	43,166		47,096
Long-term debt	275,760		271,606
Other liabilities	69,380		87,873
Total liabilities	2,158,282		2,214,838
Consumer loans
Assets of consolidated VIEs to be used to settle obligations of consolidated VIEs
Loans, net of unearned income	377,714		368,067
Allowance for credit losses on loans (ACLL)	(14,912)	(14,866)	(14,119)	(13,361)	(12,983)	(14,040)
Corporate loans
Assets of consolidated VIEs to be used to settle obligations of consolidated VIEs
Loans, net of unearned income	288,634		289,154
Allowance for credit losses on loans (ACLL)	(2,717)	$ (2,630)	(2,855)	$ (2,948)	$ (2,969)	$ (2,415)
Consolidated VIEs
Assets of consolidated VIEs to be used to settle obligations of consolidated VIEs
Cash	33		61
Trading account assets	9,990		9,153
Investments	651		594
Loans, net of unearned income	55,324		54,808
Allowance for credit losses on loans (ACLL)	(2,527)		(2,520)
Total loans, net	52,797		52,288
Other assets, at fair value	138		105
Total assets	63,609		62,201
Liabilities of consolidated VIEs for which creditors or beneficial interest holders do not have recourse to the general credit of Citigroup
Short-term borrowings	9,657		9,807
Long-term debt	7,340		10,324
Other liabilities	835		622
Total liabilities	17,832		20,753
Consolidated VIEs | Consumer loans
Assets of consolidated VIEs to be used to settle obligations of consolidated VIEs
Loans, net of unearned income	34,349		35,026
Consolidated VIEs | Corporate loans
Assets of consolidated VIEs to be used to settle obligations of consolidated VIEs
Loans, net of unearned income	$ 20,975		$ 19,782