SECURITIZATIONS AND VARIABLE INTEREST ENTITIES - Funding, Liquidity Facilities and Subordinated Interests (Details) $ in Billions	12 Months Ended
	Dec. 31, 2018 USD ($) trust	Dec. 31, 2017 USD ($)
Funding, Liquidity Facilities and Subordinated Interests
Number of trusts to hold securitized credit card receivables | trust	2
Citibank Credit Card Master Trust (Master Trust)
Funding, Liquidity Facilities and Subordinated Interests
Weighted average maturity of term notes	3 years	2 years 7 months 6 days
Term notes issued to third parties	$ 25.8	$ 27.8
Term notes retained by Citigroup affiliates	5.7	5.7
Total Trust liabilities	$ 31.5	$ 33.5
Citibank OMNI Master Trust (Omni Trust)
Funding, Liquidity Facilities and Subordinated Interests
Weighted average maturity of term notes	1 year 5 months	1 year 10 months 24 days
Term notes issued to third parties	$ 1.5	$ 1.0
Term notes retained by Citigroup affiliates	1.9	1.9
Total Trust liabilities	$ 3.4	$ 2.9