BORROWED FUNDS - Long Term Debt (Details) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
Debt Instrument [Line Items]
Long-term borrowed funds	$ 12,806	$ 15,925
Parent Company
Debt Instrument [Line Items]
Long-term borrowed funds	2,484
Parent Company | Subordinated Debt | 4.150% fixed-rate subordinated debt, due September 2022
Debt Instrument [Line Items]
Long-term borrowed funds	$ 348	348
Interest rate	4.15%
Parent Company | Subordinated Debt | 3.750% fixed-rate subordinated debt, due July 2024
Debt Instrument [Line Items]
Long-term borrowed funds	$ 250	250
Interest rate	3.75%
Parent Company | Subordinated Debt | 4.023% fixed-rate subordinated debt, due October 2024
Debt Instrument [Line Items]
Long-term borrowed funds	$ 42	42
Interest rate	4.023%
Parent Company | Subordinated Debt | 4.350% fixed-rate subordinated debt, due August 2025
Debt Instrument [Line Items]
Long-term borrowed funds	$ 249	249
Interest rate	4.35%
Parent Company | Subordinated Debt | 4.300% fixed-rate subordinated debt, due December 2025
Debt Instrument [Line Items]
Long-term borrowed funds	$ 750	749
Interest rate	4.30%
Parent Company | Senior Unsecured Notes | 2.375% fixed-rate senior unsecured debt, due July 2021
Debt Instrument [Line Items]
Long-term borrowed funds	$ 349	349
Interest rate	2.375%
Parent Company | Senior Unsecured Notes | 2.850% senior unsecured notes, due July 2026
Debt Instrument [Line Items]
Long-term borrowed funds	$ 496	0
Interest rate	2.85%
Banking and Other Subsidiaries
Debt Instrument [Line Items]
Long-term borrowed funds	$ 10,322
Banking and Other Subsidiaries | Senior Unsecured Notes | 2.500% senior unsecured notes, due 2019
Debt Instrument [Line Items]
Long-term borrowed funds	$ 0	748
Interest rate	2.50%
Banking and Other Subsidiaries | Senior Unsecured Notes | 2.450% senior unsecured notes, due 2019
Debt Instrument [Line Items]
Long-term borrowed funds	$ 749	744
Interest rate	2.45%
Banking and Other Subsidiaries | Senior Unsecured Notes | 2.250% senior unsecured notes, due 2020
Debt Instrument [Line Items]
Long-term borrowed funds	$ 699	691
Interest rate	2.25%
Banking and Other Subsidiaries | Senior Unsecured Notes | 2.678% floating-rate senior unsecured notes, due 2020
Debt Instrument [Line Items]
Long-term borrowed funds	$ 300	300
Interest rate	2.678%
Banking and Other Subsidiaries | Senior Unsecured Notes | 2.714% floating-rate senior unsecured notes, due 2020
Debt Instrument [Line Items]
Long-term borrowed funds	$ 250	250
Interest rate	2.714%
Banking and Other Subsidiaries | Senior Unsecured Notes | 2.200% senior unsecured notes, due 2020
Debt Instrument [Line Items]
Long-term borrowed funds	$ 500	499
Interest rate	2.20%
Banking and Other Subsidiaries | Senior Unsecured Notes | 2.250% senior unsecured notes, due 2020
Debt Instrument [Line Items]
Long-term borrowed funds	$ 749	738
Interest rate	2.25%
Banking and Other Subsidiaries | Senior Unsecured Notes | 2.550% senior unsecured notes, due 2021
Debt Instrument [Line Items]
Long-term borrowed funds	$ 990	964
Interest rate	2.55%
Banking and Other Subsidiaries | Senior Unsecured Notes | 3.250% Senior Unsecured Notes, Due 2022
Debt Instrument [Line Items]
Long-term borrowed funds	$ 713	0
Interest rate	3.25%
Banking and Other Subsidiaries | Senior Unsecured Notes | 2.895% floating-rate senior unsecured notes, due February 2022
Debt Instrument [Line Items]
Long-term borrowed funds	$ 299	0
Interest rate	2.895%
Banking and Other Subsidiaries | Senior Unsecured Notes | 2.954% floating-rate senior unsecured notes, due 2022
Debt Instrument [Line Items]
Long-term borrowed funds	$ 250	249
Interest rate	2.954%
Banking and Other Subsidiaries | Senior Unsecured Notes | 2.650% senior unsecured notes, due 2022
Debt Instrument [Line Items]
Long-term borrowed funds	$ 502	487
Interest rate	2.65%
Banking and Other Subsidiaries | Senior Unsecured Notes | 3.700% senior unsecured notes, due 2023
Debt Instrument [Line Items]
Long-term borrowed funds	$ 518	502
Interest rate	3.70%
Banking and Other Subsidiaries | Senior Unsecured Notes | 3.054% floating-rate senior unsecured notes, due 2023
Debt Instrument [Line Items]
Long-term borrowed funds	$ 250	249
Interest rate	3.054%
Banking and Other Subsidiaries | Senior Unsecured Notes | 3.750% Senior Unsecured Notes, Due 2026
Debt Instrument [Line Items]
Long-term borrowed funds	$ 529	0
Interest rate	3.75%
Banking and Other Subsidiaries | Federal Home Loan Bank advances
Debt Instrument [Line Items]
Long-term borrowed funds	$ 3,007	7,508
Weighted average rate	2.425%
Banking and Other Subsidiaries | Other
Debt Instrument [Line Items]
Long-term borrowed funds	$ 17	$ 9