SECURITIZATIONS AND VARIABLE INTEREST ENTITIES - Credit Card Securitizations (Details) $ in Billions	3 Months Ended
	Mar. 31, 2018 USD ($) trust	Mar. 31, 2017 USD ($)	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	$ 28.8		$ 28.8
Retained by Citigroup as trust-issued securities	7.7		7.6
Retained by Citigroup via non-certificated interests	10.1		14.4
Total ownership interests in principal amount of trust credit card receivables	46.6		$ 50.8
Credit card securitizations
Cash Flows Between Transferor and Transferee
Proceeds from new securitizations	2.8	$ 2.5
Pay down of maturing notes	$ (2.8)	$ (2.0)