BORROWED FUNDS - Short Term Debt (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Short-term Debt [Line Items]
Total short-term borrowed funds	$ 2,671	$ 4,359
Federal funds purchased
Short-term Debt [Line Items]
Total short-term borrowed funds	460	533
Securities sold under agreements to repurchase
Short-term Debt [Line Items]
Total short-term borrowed funds	355	615
Other short-term borrowed funds
Short-term Debt [Line Items]
Total short-term borrowed funds	$ 1,856	$ 3,211