SECURITIZATIONS AND VARIABLE INTEREST ENTITIES - Credit Card Securitizations (Details) $ in Billions	3 Months Ended		9 Months Ended
	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($) trust	Sep. 30, 2019 USD ($)	Dec. 31, 2019 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	$ 15.4		$ 15.4		$ 19.7
Retained by Citigroup as trust-issued securities	5.2		5.2		6.2
Retained by Citigroup via non-certificated interests	14.1		14.1		17.8
Total ownership interests in principal amount of trust credit card receivables	34.7		34.7		$ 43.7
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	$ (1.1)	$ (3.1)	$ (4.3)	$ (5.6)