SECURITIZATIONS AND VARIABLE INTEREST ENTITIES - Credit Card Securitizations (Details) $ in Billions	3 Months Ended		6 Months Ended
	Jun. 30, 2019 USD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2019 USD ($) trust	Jun. 30, 2018 USD ($)	Dec. 31, 2018 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	$ 24.8		$ 24.8		$ 27.3
Retained by Citigroup as trust-issued securities	7.2		7.2		7.6
Retained by Citigroup via non-certificated interests	11.2		11.2		11.3
Total ownership interests in principal amount of trust credit card receivables	43.2		43.2		$ 46.2
Credit card securitizations
Cash Flows Between Transferor and Transferee
Proceeds from new securitizations	0.0	$ 1.1	0.0	$ 3.9
Pay down of maturing notes	$ 0.0	$ (2.6)	$ (2.5)	$ (5.4)