SECURITIZATIONS AND VARIABLE INTEREST ENTITIES - Credit Card Securitizations (Details) $ in Billions	3 Months Ended		9 Months Ended
	Sep. 30, 2021 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2021 USD ($) trust	Sep. 30, 2020 USD ($)	Dec. 31, 2020 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	$ 11.0		$ 11.0		$ 15.7
Retained by Citigroup as trust-issued securities	7.2		7.2		7.9
Retained by Citigroup via non-certificated interests	13.9		13.9		11.1
Total ownership interests in principal amount of trust credit card receivables	32.1		32.1		$ 34.7
Credit card securitizations
Cash Flows Between Transferor and Transferee
Proceeds from new securitizations	0.0	$ 0.0	0.0	$ 0.0
Pay down of maturing notes	$ 0.0	$ (1.1)	$ (4.7)	$ (4.3)