BORROWED FUNDS - Short Term Borrowed Debt Key Data (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Federal funds purchased and securities sold under agreements to repurchase
Short-term Debt [Line Items]
Weighted-average interest rate at period end	1.39%	0.00%	1.72%
Maximum amount outstanding at month-end during the period	$ 1,134	$ 625	$ 1,282
Average amount outstanding during the period	$ 640	$ 645	$ 654
Weighted-average interest rate during the period	1.24%	0.66%	0.92%
Other short-term borrowed funds
Short-term Debt [Line Items]
Weighted-average interest rate at period end	3.03%	3.40%	2.73%
Maximum amount outstanding at month-end during the period	$ 511	$ 1,110	$ 1,110
Average amount outstanding during the period	$ 58	$ 588	$ 467
Weighted-average interest rate during the period	2.75%	1.67%	2.10%