SECURITIZATIONS AND VARIABLE INTEREST ENTITIES - Asset-Backed Commercial Paper Conduits (Details) - USD ($)	9 Months Ended	12 Months Ended
	Sep. 30, 2021	Dec. 31, 2020
Classification of Other Securitization Details
Commercial paper	$ 17,012,000,000	$ 18,010,000,000
Citi-administered asset-backed commercial paper conduits
Classification of Other Securitization Details
Purchased assets outstanding under conduits	12,500,000,000	16,700,000,000
Incremental funding commitments with clients	$ 20,500,000,000	$ 17,100,000,000
Weighted average life of commercial paper issued by conduits	65 days	54 days
Citi-administered asset-backed commercial paper conduits | Minimum
Classification of Other Securitization Details
Letters of credit as percentage of conduit assets	8.00%
Floor price of conduit's assets	$ 200,000,000
Citi-administered asset-backed commercial paper conduits | Maximum
Classification of Other Securitization Details
Letters of credit as percentage of conduit assets	10.00%
Citi-administered asset-backed consolidated commercial paper conduits (ABCP)
Classification of Other Securitization Details
Letters of credit provided to conduits	$ 1,100,000,000	$ 1,500,000,000
Commercial paper	$ 2,500,000,000	$ 6,600,000,000