BORROWED FUNDS - Short Term Borrowed Debt Key Data (Details) - USD ($) $ in Millions		3 Months Ended		12 Months Ended
		Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Federal funds purchased and securities sold under agreements to repurchase
Short-term Debt [Line Items]
Weighted-average interest rate at period end	[1]	0.00%	0.43%	0.74%
Maximum amount outstanding at month-end during the period	[2]	$ 625	$ 1,174	$ 1,174
Average amount outstanding during the period	[2]	$ 645	$ 882	$ 776
Weighted-average interest rate during the period	[1]	0.66%	0.22%	0.36%
Other short-term borrowed funds
Short-term Debt [Line Items]
Weighted-average interest rate at period end	[1]	2.41%	1.08%	1.72%
Maximum amount outstanding at month-end during the period		$ 1,853	$ 3,508	$ 3,508
Average amount outstanding during the period		$ 1,481	$ 2,963	$ 2,321
Weighted-average interest rate during the period	[1]	2.14%	1.08%	1.32%

[1] Rates exclude certain hedging costs.

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