DEBT - Short-Term Borrowings (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Short-term Debt [Line Items]
Commercial paper	$ 15,034	$ 13,238
Other borrowings	24,288	19,108
Total short-term borrowings	39,322	32,346
Collateralized short-term advances from Federal Home Loan Bank	13,300	9,500
Bank
Short-term Debt [Line Items]
Commercial paper	13,060	13,238
Broker-dealer and other
Short-term Debt [Line Items]
Commercial paper	$ 1,974	$ 0