BORROWED FUNDS - Narrative (Details) - USD ($) $ in Millions	Dec. 31, 2017		Dec. 31, 2016
Debt Instrument [Line Items]
Hedging basis adjustments	$ 693	[1]	$ 759
Short-term borrowed funds	2,671		4,359
FHLB advances and letters of credit | Secured Debt
Debt Instrument [Line Items]
Short-term borrowed funds	9,400		13,400
FHLB advances
Debt Instrument [Line Items]
Available borrowing capacity	8,000		2,800
FRB advances
Debt Instrument [Line Items]
Available borrowing capacity	41,200
Parent company
Debt Instrument [Line Items]
Principal balance	2,300		2,300
Unamortized deferred issuance costs and/or discounts	(5)		(7)
Banking Subsidiaries
Debt Instrument [Line Items]
Principal balance	9,500		10,500
Unamortized deferred issuance costs and/or discounts	(19)		(12)
Hedging basis adjustments	$ (63)		$ (40)

[1] Amounts reflect changes in the treatment of variation margin on certain centrally cleared derivatives.