BORROWED FUNDS - Short Term Borrowed Debt Key Data (Details) - USD ($) $ in Millions		3 Months Ended		12 Months Ended
		Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016
Federal funds purchased and securities sold under agreements to repurchase
Short-term Debt [Line Items]
Weighted-average interest rate at period end	[1]	0.43%	0.01%	0.26%
Maximum amount outstanding at month-end during the period	[2]	$ 1,174	$ 1,274	$ 1,522
Average amount outstanding during the period	[2]	$ 882	$ 881	$ 947
Weighted-average interest rate during the period	[1]	0.22%	0.06%	0.09%
Other short-term borrowed funds (primarily current portion of FHLB advances)
Short-term Debt [Line Items]
Weighted-average interest rate at period end	[1]	1.08%	0.57%	0.94%
Maximum amount outstanding at month-end during the period		$ 3,508	$ 3,300	$ 5,461
Average amount outstanding during the period		$ 2,963	$ 3,098	$ 3,207
Weighted-average interest rate during the period	[1]	1.08%	0.58%	0.64%

[1] Rates exclude certain hedging costs.

[2] Balances are net of certain short-term receivables associated with reverse repurchase agreements, as applicable.