BORROWED FUNDS - Short Term Borrowed Debt Key Data (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended		12 Months Ended
		Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 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%	0.00%	0.00%	0.00%	0.74%
Maximum amount outstanding at month-end during the period	[2]	$ 382	$ 724	$ 1,045	$ 1,174	$ 1,174
Average amount outstanding during the period	[2]	$ 643	$ 733	$ 598	$ 807	$ 776
Weighted-average interest rate during the period	[1]	0.91%	0.47%	0.76%	0.34%	0.36%
Other short-term borrowed funds
Short-term Debt [Line Items]
Weighted-average interest rate at period end	[1]	2.41%	1.47%	2.41%	1.47%	1.72%
Maximum amount outstanding at month-end during the period		$ 2,502	$ 1,755	$ 2,502	$ 3,508	$ 3,508
Average amount outstanding during the period		$ 2,239	$ 1,624	$ 1,802	$ 2,283	$ 2,321
Weighted-average interest rate during the period	[1]	2.45%	1.48%	2.38%	1.22%	1.32%

[1] Rates exclude certain hedging costs.

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