BORROWED FUNDS - Short Term Borrowed Debt Key Data (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	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	0.03%	0.25%	0.03%	0.25%	0.15%
Maximum amount outstanding at month-end during the period	$ 1,032	$ 3,356	$ 1,274	$ 5,375	$ 5,375
Average amount outstanding during the period	$ 910	$ 2,880	$ 922	$ 3,947	$ 3,364
Weighted-average interest rate during the period	0.10%	0.32%	0.09%	0.22%	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	0.63%	0.29%	0.63%	0.29%	0.44%
Maximum amount outstanding at month-end during the period	$ 2,515	$ 5,861	$ 4,764	$ 7,004	$ 7,004
Average amount outstanding during the period	$ 2,564	$ 5,062	$ 3,133	$ 6,169	$ 5,865
Weighted-average interest rate during the period	0.63%	0.29%	0.62%	0.27%	0.28%