BORROWED FUNDS - Short Term Borrowed Debt Key Data (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 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	1.93%	0.00%	1.93%	0.00%	1.72%
Maximum amount outstanding at month-end during the period	$ 1,499	$ 1,045	$ 1,499	$ 1,045	$ 1,282
Average amount outstanding during the period	$ 818	$ 504	$ 729	$ 574	$ 654
Weighted-average interest rate during the period	1.76%	0.71%	1.54%	0.68%	0.92%
Other short-term borrowed funds
Short-term Debt [Line Items]
Weighted-average interest rate at period end	2.47%	3.22%	2.47%	3.22%	2.73%
Maximum amount outstanding at month-end during the period	$ 508	$ 1,110	$ 511	$ 1,110	$ 1,110
Average amount outstanding during the period	$ 45	$ 191	$ 52	$ 388	$ 467
Weighted-average interest rate during the period	2.66%	1.90%	2.71%	1.73%	2.10%