BORROWED FUNDS - Short Term Borrowed Debt Key Data (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	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	0.41%	1.72%	0.74%
Maximum amount outstanding at month-end during the period	$ 1,499	$ 1,282	$ 1,174
Average amount outstanding during the period	$ 599	$ 654	$ 776
Weighted-average interest rate during the period	1.36%	0.92%	0.36%
Other short-term borrowed funds
Short-term Debt [Line Items]
Weighted-average interest rate at period end	3.85%	2.73%	1.33%
Maximum amount outstanding at month-end during the period	$ 511	$ 1,110	$ 2,759
Average amount outstanding during the period	$ 66	$ 467	$ 1,571
Weighted-average interest rate during the period	2.50%	2.10%	1.09%