BORROWED FUNDS - Long Term Debt (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Debt Instrument [Line Items]
Long-term borrowed funds	$ 11,725	$ 15,925
Parent Company
Debt Instrument [Line Items]
Long-term borrowed funds	1,988
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%
Banking and Other Subsidiaries
Debt Instrument [Line Items]
Long-term borrowed funds	$ 9,737
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	$ 745	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	$ 694	691
Interest rate	2.25%
Banking and Other Subsidiaries | Senior Unsecured Notes | 3.155% floating-rate senior unsecured notes, due 2020
Debt Instrument [Line Items]
Long-term borrowed funds	$ 300	300
Interest rate	3.155%
Banking and Other Subsidiaries | Senior Unsecured Notes | 3.216% floating-rate senior unsecured notes, due 2020
Debt Instrument [Line Items]
Long-term borrowed funds	$ 250	250
Interest rate	3.216%
Banking and Other Subsidiaries | Senior Unsecured Notes | 2.200% senior unsecured notes, due 2020
Debt Instrument [Line Items]
Long-term borrowed funds	$ 499	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	$ 742	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	$ 973	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	$ 702	0
Interest rate	3.25%
Banking and Other Subsidiaries | Senior Unsecured Notes | 3.413% floating-rate senior unsecured notes, due February 2022
Debt Instrument [Line Items]
Long-term borrowed funds	$ 299	0
Interest rate	3.413%
Banking and Other Subsidiaries | Senior Unsecured Notes | 3.456% floating-rate senior unsecured notes, due 2022
Debt Instrument [Line Items]
Long-term borrowed funds	$ 249	249
Interest rate	3.456%
Banking and Other Subsidiaries | Senior Unsecured Notes | 2.650% senior unsecured notes, due 2022
Debt Instrument [Line Items]
Long-term borrowed funds	$ 492	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	$ 508	502
Interest rate	3.70%
Banking and Other Subsidiaries | Senior Unsecured Notes | 3.551% floating-rate senior unsecured notes, due 2023
Debt Instrument [Line Items]
Long-term borrowed funds	$ 249	249
Interest rate	3.551%
Banking and Other Subsidiaries | Senior Unsecured Notes | 3.750% Senior Unsecured Notes, Due 2026
Debt Instrument [Line Items]
Long-term borrowed funds	$ 506	0
Interest rate	3.75%
Banking and Other Subsidiaries | Federal Home Loan Bank advances
Debt Instrument [Line Items]
Long-term borrowed funds	$ 2,508	7,508
Weighted average rate	2.787%
Banking and Other Subsidiaries | Other
Debt Instrument [Line Items]
Long-term borrowed funds	$ 21	$ 9