SECURITIZATIONS AND VARIABLE INTEREST ENTITIES - Credit Card Securitizations (Details) $ in Billions	3 Months Ended		9 Months Ended
	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($) trust	Sep. 30, 2017 USD ($)	Dec. 31, 2017 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	$ 26.3		$ 26.3		$ 28.8
Retained by Citigroup as trust-issued securities	7.5		7.5		7.6
Retained by Citigroup via non-certificated interests	11.6		11.6		14.4
Total ownership interests in principal amount of trust credit card receivables	45.4		45.4		$ 50.8
Credit card securitizations
Cash Flows Between Transferor and Transferee
Proceeds from new securitizations	1.9	$ 2.2	5.8	$ 9.8
Pay down of maturing notes	$ (2.9)	$ (1.8)	$ (8.3)	$ (4.6)