SECURITIZATIONS AND VARIABLE INTEREST ENTITIES - Credit Card Securitizations (Details)	12 Months Ended
	Dec. 31, 2023 USD ($) trust	Dec. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)
Securitized credit card receivables
Number of trusts to hold securitized credit card receivables | trust	2
Ownership interests in principal amount of trust credit card receivables
Sold to investors via trust-issued securities	$ 6,900,000,000	$ 7,900,000,000
Retained by Citigroup as trust-issued securities	5,100,000,000	6,400,000,000
Retained by Citigroup via non-certificated interests	21,400,000,000	19,500,000,000
Total ownership interests in principal amount of trust credit card receivables	33,400,000,000	33,800,000,000
Credit card securitizations
Securitized credit card receivables
Gains (losses) recognized on the securitization	0
Cash Flows Between Transferor and Transferee
Proceeds from new securitizations	1,500,000,000	300,000,000	$ 0
Pay down of maturing notes	$ (2,400,000,000)	$ (2,100,000,000)	$ (6,000,000,000.0)