SECURITIZATIONS AND VARIABLE INTEREST ENTITIES - Asset-Backed Commercial Paper Conduits (Details) - USD ($)	6 Months Ended	12 Months Ended
	Jun. 30, 2019	Dec. 31, 2018
Classification of Other Securitization Details
Commercial paper	$ 17,126,000,000	$ 13,238,000,000
Citi-administered asset-backed commercial paper conduits
Classification of Other Securitization Details
Purchased assets outstanding under conduits	16,400,000,000	18,800,000,000
Incremental funding commitments with clients	$ 16,300,000,000	$ 14,000,000,000.0
Weighted average life of commercial paper issued by conduits	43 days	53 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	$ 1,500,000,000	$ 1,700,000,000
Commercial paper	$ 3,500,000,000	$ 5,500,000,000