BORROWED FUNDS - Short Term Borrowed Debt Key Data (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Federal funds purchased and securities sold under agreements to repurchase
Short-term Debt [Line Items]
Weighted-average interest rate at period end	0.15%	0.14%	0.09%
Maximum amount outstanding at month-end during the period	$ 5,375	$ 7,022	$ 5,114
Average amount outstanding during the period	$ 3,364	$ 5,699	$ 2,400
Weighted-average interest rate during the period	0.22%	0.12%	0.31%
Other short-term borrowed funds (primarily current portion of FHLB advances)
Short-term Debt [Line Items]
Weighted-average interest rate at period end	0.44%	0.26%	0.20%
Maximum amount outstanding at month-end during the period	$ 7,004	$ 7,702	$ 2,251
Average amount outstanding during the period	$ 5,865	$ 5,640	$ 251
Weighted-average interest rate during the period	0.28%	0.25%	0.44%