SECURITIZATIONS AND VARIABLE INTEREST ENTITIES - Assets and Liabilities of VIEs (Details) - USD ($) $ in Millions	Sep. 30, 2024	Jun. 30, 2024	Dec. 31, 2023	Sep. 30, 2023	Jun. 30, 2023	Dec. 31, 2022
Assets of consolidated VIEs to be used to settle obligations of consolidated VIEs
Cash and due from banks	$ 25,266		$ 27,342	$ 26,548
Trading account assets	458,072		411,756
Investments	490,671		519,085
Loans, net of unearned income	688,922		689,362
Allowance for credit losses on loans (ACLL)	(18,356)	$ (18,216)	(18,145)	(17,629)	$ (17,496)	$ (16,974)
Total loans, net	670,566		671,217
Other assets, at fair value	104,771		95,963
Total assets	2,430,663		2,411,834
Liabilities of consolidated VIEs for which creditors or beneficial interest holders do not have recourse to the general credit of Citigroup
Short-term borrowings	41,340		37,457
Long-term debt	299,081		286,619
Other liabilities	68,244		75,835
Total liabilities	2,220,761		2,205,583
Consumer loans
Assets of consolidated VIEs to be used to settle obligations of consolidated VIEs
Loans, net of unearned income	389,151		389,197
Allowance for credit losses on loans (ACLL)	(15,765)	(15,732)	(15,431)	(14,912)	(14,866)	(14,119)
Corporate loans
Assets of consolidated VIEs to be used to settle obligations of consolidated VIEs
Loans, net of unearned income	299,771		300,165
Allowance for credit losses on loans (ACLL)	(2,591)	$ (2,484)	(2,714)	$ (2,717)	$ (2,630)	$ (2,855)
Consolidated VIEs
Assets of consolidated VIEs to be used to settle obligations of consolidated VIEs
Cash and due from banks	89		44
Trading account assets	6,153		11,350
Investments	877		767
Loans, net of unearned income	52,662		56,348
Allowance for credit losses on loans (ACLL)	(2,385)		(2,481)
Total loans, net	50,277		53,867
Other assets, at fair value	166		160
Total assets	57,562		66,188
Liabilities of consolidated VIEs for which creditors or beneficial interest holders do not have recourse to the general credit of Citigroup
Short-term borrowings	9,963		9,692
Long-term debt	5,916		8,443
Other liabilities	1,366		927
Total liabilities	17,245		19,062
Consolidated VIEs | Consumer loans
Assets of consolidated VIEs to be used to settle obligations of consolidated VIEs
Loans, net of unearned income	32,229		35,141
Consolidated VIEs | Corporate loans
Assets of consolidated VIEs to be used to settle obligations of consolidated VIEs
Loans, net of unearned income	$ 20,433		$ 21,207