BORROWED FUNDS - Short Term Borrowed Debt Key Data (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2015	Dec. 31, 2014
Federal funds purchased and securities sold under agreements to repurchase
Short-term Debt [Line Items]
Weighted-average interest rate at period end	0.25%	0.14%
Maximum amount outstanding at month-end during the period	$ 5,375	$ 7,022
Average amount outstanding during the period	$ 3,947	$ 5,699
Weighted-average interest rate during the period	0.22%	0.12%
Other short-term borrowed funds (primarily current portion of FHLB advances)
Short-term Debt [Line Items]
Weighted-average interest rate at period end	0.29%	0.26%
Maximum amount outstanding at month-end during the period	$ 7,004	$ 7,702
Average amount outstanding during the period	$ 6,169	$ 5,640
Weighted-average interest rate during the period	0.27%	0.25%