SECURITIZATIONS AND VARIABLE INTEREST ENTITIES - Credit Card Securitizations (Details) $ in Billions	3 Months Ended		9 Months Ended
	Sep. 30, 2016 USD ($)	Sep. 30, 2015 USD ($)	Sep. 30, 2016 USD ($) trust	Sep. 30, 2015 USD ($)	Dec. 31, 2015 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	$ 23.4		$ 23.4		$ 29.7
Retained by Citigroup as trust-issued securities	7.8		7.8		9.4
Retained by Citigroup via non-certificated interests	18.7		18.7		16.5
Total ownership interests in principal amount of trust credit card receivables	49.9		49.9		$ 55.6
Credit card securitizations
Cash Flows Between Transferor and Transferee
Proceeds from new securitizations	0.0	$ 0.0	0.0	$ 0.0
Pay down of maturing notes	$ (2.8)	$ (0.7)	$ (6.3)	$ (6.5)