SECURITIZATIONS AND VARIABLE INTEREST ENTITIES - Funding, Liquidity Facilities and Subordinated Interests (Details) - USD ($) $ in Billions	3 Months Ended	12 Months Ended
	Mar. 31, 2021	Dec. 31, 2020
Citibank Credit Card Master Trust (Master Trust)
Funding, Liquidity Facilities and Subordinated Interests
Weighted average maturity of term notes	3 years 6 months	2 years 10 months 24 days
Term notes issued to third parties	$ 10.6	$ 13.9
Term notes retained by Citigroup affiliates	2.6	2.7
Total Master Trust liabilities	$ 13.2	$ 16.6
Citibank OMNI Master Trust (Omni Trust)
Funding, Liquidity Facilities and Subordinated Interests
Weighted average maturity of term notes	10 months 24 days	1 year 1 month 6 days
Term notes issued to third parties	$ 1.5	$ 1.8
Term notes retained by Citigroup affiliates	5.0	5.2
Total Master Trust liabilities	$ 6.5	$ 7.0