BORROWED FUNDS - Long Term Debt (Details) - USD ($) $ in Millions		Mar. 31, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Long-term borrowed funds		$ 13,486	$ 11,765
Banking Subsidiaries
Debt Instrument [Line Items]
Long-term borrowed funds		11,166
Banking Subsidiaries | Senior Unsecured Notes | 2.450% senior unsecured notes, due 2019
Debt Instrument [Line Items]
Long-term borrowed funds	[1]	$ 738	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	[1],[2]	$ 0	741
Interest rate		2.50%
Banking Subsidiaries | Senior Unsecured Notes | 2.25% Senior Unsecured Notes, Due 2020, Settlement Date of March 02, 2017 [Member]
Debt Instrument [Line Items]
Long-term borrowed funds	[1]	$ 688	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	[1]	$ 299	299
Banking Subsidiaries | Senior Unsecured Notes | Floating-rate senior unsecured notes, due 2020
Debt Instrument [Line Items]
Long-term borrowed funds	[1]	250	249
Banking Subsidiaries | Senior Unsecured Notes | 2.200% senior unsecured notes, due 2020
Debt Instrument [Line Items]
Long-term borrowed funds	[1]	$ 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	[1]	$ 734	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	[1]	$ 953	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	[1]	$ 249	249
Banking Subsidiaries | Senior Unsecured Notes | 2.650% senior unsecured notes, due 2022
Debt Instrument [Line Items]
Long-term borrowed funds	[1]	$ 482	491
Interest rate		2.65%
Banking Subsidiaries | Senior Unsecured Notes | 3.700% senior unsecured notes, due 2023 [Member]
Debt Instrument [Line Items]
Long-term borrowed funds	[1]	$ 500	0
Interest rate		3.70%
Banking Subsidiaries | Senior Unsecured Notes | Floating-rate senior unsecured notes, due 2023 [Member]
Debt Instrument [Line Items]
Long-term borrowed funds	[1]	$ 249	0
Banking Subsidiaries | Federal Home Loan advances
Debt Instrument [Line Items]
Long-term borrowed funds		5,511	3,761
Banking Subsidiaries | Other
Debt Instrument [Line Items]
Long-term borrowed funds		14	16
Parent Company
Debt Instrument [Line Items]
Long-term borrowed funds		2,320
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 subordinated debt, due 2023, converting to floating at 3-month LIBOR 3.56% and callable beginning June 2018
Debt Instrument [Line Items]
Long-term borrowed funds		$ 333	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 | Subordinated Debt | LIBOR | 5.158% fixed-to-floating rate subordinated debt, due 2023, converting to floating at 3-month LIBOR 3.56% and callable beginning June 2018
Debt Instrument [Line Items]
Interest rate		3.56%
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%

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

[2] Reclassified to short-term borrowed funds.