SECURITIZATIONS AND VARIABLE INTEREST ENTITIES - Funding, Liquidity Facilities and Subordinated Interests (Details) - USD ($) $ in Billions	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Citibank Credit Card Master Trust (Master Trust)
Funding, Liquidity Facilities and Subordinated Interests
Weighted average maturity of term notes	3 years	2 years 6 months 25 days
Term notes issued to third parties	$ 24.8	$ 27.8
Term notes retained by Citigroup affiliates	5.7	5.7
Total Master Trust liabilities	$ 30.5	$ 33.5
Citibank OMNI Master Trust (Omni Trust)
Funding, Liquidity Facilities and Subordinated Interests
Weighted average maturity of term notes	1 year 7 months 30 days	1 year 11 months
Term notes issued to third parties	$ 1.5	$ 1.0
Term notes retained by Citigroup affiliates	1.9	1.9
Total Master Trust liabilities	$ 3.4	$ 2.9