BORROWED FUNDS - Short Term Borrowed Debt Key Data (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2017	Dec. 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	[1]	0.74%	0.26%	0.15%
Maximum amount outstanding at month-end during the period	[2]	$ 1,174	$ 1,522	$ 5,375
Average amount outstanding during the period	[2]	$ 776	$ 947	$ 3,364
Weighted-average interest rate during the period	[1]	0.36%	0.09%	0.22%
Other short-term borrowed funds
Short-term Debt [Line Items]
Weighted-average interest rate at period end	[1]	1.72%	0.94%	0.44%
Maximum amount outstanding at month-end during the period		$ 3,508	$ 5,461	$ 7,004
Average amount outstanding during the period		$ 2,321	$ 3,207	$ 5,865
Weighted-average interest rate during the period	[1]	1.32%	0.64%	0.28%

[1] Rates exclude certain hedging costs.

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