SECURITIZATIONS AND VARIABLE INTEREST ENTITIES - Credit Card Securitizations (Details) $ in Billions	3 Months Ended		6 Months Ended
	Jun. 30, 2022 USD ($)	Jun. 30, 2021 USD ($)	Jun. 30, 2022 USD ($) trust	Jun. 30, 2021 USD ($)	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	$ 9.7		$ 9.7		$ 9.7
Retained by Citigroup as trust-issued securities	6.5		6.5		7.2
Retained by Citigroup via non-certificated interests	16.9		16.9		16.1
Total ownership interests in principal amount of trust credit card receivables	33.1		33.1		$ 33.0
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)	$ 0.0	$ (4.7)