BORROWED FUNDS - Short Term Borrowed Debt Key Data (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2016	Dec. 31, 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	[1]	0.26%	0.15%	0.14%
Maximum amount outstanding at month-end during the period	[2]	$ 1,522	$ 5,375	$ 7,022
Average amount outstanding during the period	[2]	$ 947	$ 3,364	$ 5,699
Weighted-average interest rate during the period	[1]	0.09%	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	[1]	0.94%	0.44%	0.26%
Maximum amount outstanding at month-end during the period		$ 5,461	$ 7,004	$ 7,702
Average amount outstanding during the period		$ 3,207	$ 5,865	$ 5,640
Weighted-average interest rate during the period	[1]	0.64%	0.28%	0.25%

[1] Rates exclude certain hedging costs.

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