SECURITIZATIONS AND VARIABLE INTEREST ENTITIES - Credit Card Securitizations (Details) $ in Billions	3 Months Ended		9 Months Ended
	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)	Sep. 30, 2017 USD ($) trust	Sep. 30, 2016 USD ($)	Dec. 31, 2016 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	$ 28.0		$ 28.0		$ 22.7
Retained by Citigroup as trust-issued securities	9.2		9.2		7.4
Retained by Citigroup via non-certificated interests	12.5		12.5		20.6
Total ownership interests in principal amount of trust credit card receivables	49.7		49.7		$ 50.7
Credit card securitizations
Cash Flows Between Transferor and Transferee
Proceeds from new securitizations	2.2	$ 0.0	9.8	$ 0.0
Pay down of maturing notes	$ (1.8)	$ (2.8)	$ (4.6)	$ (6.3)