SECURITIZATIONS AND VARIABLE INTEREST ENTITIES - Credit Card Securitizations (Details) $ in Billions	3 Months Ended
	Mar. 31, 2019 USD ($) trust	Mar. 31, 2018 USD ($)	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		$ 27.3
Retained by Citigroup as trust-issued securities	7.6		7.6
Retained by Citigroup via non-certificated interests	10.5		11.3
Total ownership interests in principal amount of trust credit card receivables	42.9		$ 46.2
Credit card securitizations
Cash Flows Between Transferor and Transferee
Proceeds from new securitizations	0.0	$ 2.8
Pay down of maturing notes	$ (2.5)	$ (2.8)