SECURITIZATIONS AND VARIABLE INTEREST ENTITIES - Asset-Backed Commercial Paper Conduits (Details) - USD ($)	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Classification of Other Securitization Details
Commercial paper	$ 12,051,000,000	$ 9,940,000,000
Citi-administered asset-backed commercial paper conduits (ABCP)
Classification of Other Securitization Details
Purchased assets outstanding under conduits	17,400,000,000	19,300,000,000
Incremental funding commitments with clients	$ 16,300,000,000	$ 14,500,000,000
Weighted average life of commercial paper issued by conduits	55 days	51 days
Citi-administered asset-backed commercial paper conduits (ABCP) | 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 (ABCP) | 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	$ 1,600,000,000	$ 1,600,000,000
Commercial paper	$ 5,400,000,000	$ 9,300,000,000