BORROWED FUNDS - Long Term Debt (Details) - USD ($) $ in Millions		Sep. 30, 2018	Jun. 29, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Long-term borrowed funds		$ 15,639		$ 11,765
Parent Company
Debt Instrument [Line Items]
Long-term borrowed funds		1,987
Parent Company | Subordinated Debt | 4.150% fixed-rate subordinated debt, due 2022
Debt Instrument [Line Items]
Long-term borrowed funds		$ 348		348
Interest rate		4.15%
Parent Company | Subordinated Debt | 5.158% fixed-to-floating rate callable subordinated debt
Debt Instrument [Line Items]
Long-term borrowed funds	[1]	$ 0		333
Interest rate			5.158%
Parent Company | Subordinated Debt | 3.750% fixed-rate subordinated debt, due 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 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 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 2025
Debt Instrument [Line Items]
Long-term borrowed funds		$ 749		749
Interest rate		4.30%
Parent Company | Senior Unsecured Notes | 2.375% fixed-rate senior unsecured debt, due 2021
Debt Instrument [Line Items]
Long-term borrowed funds		$ 349		349
Interest rate		2.375%
Banking Subsidiaries
Debt Instrument [Line Items]
Long-term borrowed funds		$ 13,652
Banking Subsidiaries | Senior Unsecured Notes | 2.450% senior unsecured notes, due 2019
Debt Instrument [Line Items]
Long-term borrowed funds	[2]	$ 742		743
Interest rate		2.45%
Banking Subsidiaries | Senior Unsecured Notes | 2.500% senior unsecured notes, due 2019
Debt Instrument [Line Items]
Long-term borrowed funds	[2],[3]	$ 0		741
Interest rate		2.50%
Banking Subsidiaries | Senior Unsecured Notes | 2.250% senior unsecured notes, due 2020
Debt Instrument [Line Items]
Long-term borrowed funds	[2]	$ 687		692
Interest rate		2.25%
Banking Subsidiaries | Senior Unsecured Notes | Floating-rate senior unsecured notes, due 2020
Debt Instrument [Line Items]
Long-term borrowed funds	[2]	$ 300		299
Banking Subsidiaries | Senior Unsecured Notes | Floating-rate senior unsecured notes, due 2020
Debt Instrument [Line Items]
Long-term borrowed funds	[2]	250		249
Banking Subsidiaries | Senior Unsecured Notes | 2.200% senior unsecured notes, due 2020
Debt Instrument [Line Items]
Long-term borrowed funds	[2]	$ 499		498
Interest rate		2.20%
Banking Subsidiaries | Senior Unsecured Notes | 2.250% senior unsecured notes, due 2020
Debt Instrument [Line Items]
Long-term borrowed funds	[2]	$ 731		742
Interest rate		2.25%
Banking Subsidiaries | Senior Unsecured Notes | 2.550% senior unsecured notes, due 2021
Debt Instrument [Line Items]
Long-term borrowed funds	[2]	$ 951		964
Interest rate		2.55%
Banking Subsidiaries | Senior Unsecured Notes | Floating-rate senior unsecured notes, due 2022
Debt Instrument [Line Items]
Long-term borrowed funds	[2]	$ 249		249
Banking Subsidiaries | Senior Unsecured Notes | 2.650% senior unsecured notes, due 2022
Debt Instrument [Line Items]
Long-term borrowed funds	[2]	$ 478		491
Interest rate		2.65%
Banking Subsidiaries | Senior Unsecured Notes | 3.700% senior unsecured notes, due 2023
Debt Instrument [Line Items]
Long-term borrowed funds	[2]	$ 492		0
Interest rate		3.70%
Banking Subsidiaries | Senior Unsecured Notes | Floating-rate senior unsecured notes, due 2023
Debt Instrument [Line Items]
Long-term borrowed funds	[2]	$ 249		0
Banking Subsidiaries | Federal Home Loan Bank advances
Debt Instrument [Line Items]
Long-term borrowed funds		8,012		3,761
Banking Subsidiaries | Other
Debt Instrument [Line Items]
Long-term borrowed funds		$ 12		$ 16

[1] Redeemed on June 29, 2018.

[2] Issued under CBNA’s Global Bank Note Program.

[3] Reclassified to short-term borrowed funds.