BORROWED FUNDS - Short Term Borrowed Debt Key Data (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2016	Dec. 31, 2015
Federal funds purchased and securities sold under agreements to repurchase
Short-term Debt [Line Items]
Weighted-average interest rate at period end	0.01%	0.15%
Maximum amount outstanding at month-end during the period	$ 1,274	$ 5,375
Average amount outstanding during the period	$ 881	$ 3,364
Weighted-average interest rate during the period	0.06%	0.22%
Other short-term borrowed funds (primarily current portion of FHLB advances)
Short-term Debt [Line Items]
Weighted-average interest rate at period end	0.57%	0.44%
Maximum amount outstanding at month-end during the period	$ 3,300	$ 7,004
Average amount outstanding during the period	$ 3,098	$ 5,865
Weighted-average interest rate during the period	0.58%	0.28%