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DEBT - Long-Term Debt (Details) - USD ($)
$ in Millions
Sep. 30, 2017
Dec. 31, 2016
Debt Instrument    
Long-term debt, at fair value $ 232,673 $ 206,178
Citigroup Inc.    
Debt Instrument    
Long-term debt, at fair value 151,914 147,333
Bank    
Debt Instrument    
Long-term debt, at fair value 62,078 49,454
Bank | Senior notes    
Debt Instrument    
Collateralized long-term advances from Federal Home Loan Bank 19,800 21,600
Broker-dealer    
Debt Instrument    
Long-term debt, at fair value $ 18,681 $ 9,391