BORROWED FUNDS - Long Term Debt (Details) - USD ($) $ in Millions		Dec. 31, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Long-term borrowed funds		$ 14,433	$ 11,765
5.158% fixed-to-floating rate subordinated debt, callable, due 2023
Debt Instrument [Line Items]
Interest rate		5.158%
Parent company
Debt Instrument [Line Items]
Long-term borrowed funds		$ 1,987
Banking Subsidiaries
Debt Instrument [Line Items]
Long-term borrowed funds		$ 12,446
Senior Unsecured Notes | Parent company | 2.375% fixed rate senior unsecured debt, due 2021
Debt Instrument [Line Items]
Interest rate		2.375%
Long-term borrowed funds		$ 349	349
Senior Unsecured Notes | Banking Subsidiaries | 2.450% senior unsecured notes, due 2019
Debt Instrument [Line Items]
Interest rate		2.45%
Long-term borrowed funds	[1],[2]	$ 0	743
Senior Unsecured Notes | Banking Subsidiaries | 2.500% senior unsecured notes, due 2019
Debt Instrument [Line Items]
Interest rate		2.50%
Long-term borrowed funds	[1],[2]	$ 0	741
Senior Unsecured Notes | Banking Subsidiaries | 2.250% senior unsecured notes, due 2020 (1)
Debt Instrument [Line Items]
Interest rate		2.25%
Long-term borrowed funds	[1]	$ 691	692
Senior Unsecured Notes | Banking Subsidiaries | 3.278% floating rate senior unsecured notes, due 2020
Debt Instrument [Line Items]
Interest rate		3.278%
Long-term borrowed funds	[1],[3]	$ 300	299
Senior Unsecured Notes | Banking Subsidiaries | 3.247% floating rate senior unsecured notes, due 2020
Debt Instrument [Line Items]
Interest rate		3.247%
Long-term borrowed funds	[1],[3]	$ 250	249
Senior Unsecured Notes | Banking Subsidiaries | 2.200% senior unsecured notes, due 2020
Debt Instrument [Line Items]
Interest rate		2.20%
Long-term borrowed funds	[1]	$ 499	498
Senior Unsecured Notes | Banking Subsidiaries | 2.250% senior unsecured notes, due 2020
Debt Instrument [Line Items]
Interest rate		2.25%
Long-term borrowed funds	[1]	$ 738	742
Senior Unsecured Notes | Banking Subsidiaries | 2.550% senior unsecured notes, due 2021
Debt Instrument [Line Items]
Interest rate		2.55%
Long-term borrowed funds	[1]	$ 964	964
Senior Unsecured Notes | Banking Subsidiaries | 3.487% floating-rate senior unsecured notes, due 2022
Debt Instrument [Line Items]
Interest rate		3.487%
Long-term borrowed funds	[1],[3]	$ 249	249
Senior Unsecured Notes | Banking Subsidiaries | 2.650% senior unsecured notes, due 2022
Debt Instrument [Line Items]
Interest rate		2.65%
Long-term borrowed funds	[1]	$ 487	491
Senior Unsecured Notes | Banking Subsidiaries | 3.700% senior unsecured notes, due 2023
Debt Instrument [Line Items]
Interest rate		3.70%
Long-term borrowed funds	[1]	$ 502	0
Senior Unsecured Notes | Banking Subsidiaries | 3.753% floating-rate senior unsecured notes, due 2023
Debt Instrument [Line Items]
Interest rate		3.753%
Long-term borrowed funds	[1],[3]	$ 249	0
Subordinated Debt | Parent company | 4.150% fixed rate subordinated debt, due 2022
Debt Instrument [Line Items]
Interest rate		4.15%
Long-term borrowed funds		$ 348	348
Subordinated Debt | Parent company | 5.158% fixed-to-floating rate subordinated debt, callable, due 2023
Debt Instrument [Line Items]
Interest rate		5.158%
Long-term borrowed funds	[4]	$ 0	333
Subordinated Debt | Parent company | 3.750% fixed rate subordinated debt due 2024
Debt Instrument [Line Items]
Interest rate		3.75%
Long-term borrowed funds		$ 250	250
Subordinated Debt | Parent company | 4.023% fixed rate subordinated debt, due 2024
Debt Instrument [Line Items]
Interest rate		4.023%
Long-term borrowed funds		$ 42	42
Subordinated Debt | Parent company | 4.350% fixed rate subordinated debt, due 2025
Debt Instrument [Line Items]
Interest rate		4.35%
Long-term borrowed funds		$ 249	249
Subordinated Debt | Parent company | 4.300% fixed rate subordinated debt, due 2025
Debt Instrument [Line Items]
Interest rate		4.30%
Long-term borrowed funds		$ 749	749
Federal Home Loan advances | Banking Subsidiaries
Debt Instrument [Line Items]
Weighted average interest rate		2.725%
Long-term borrowed funds		$ 7,508	3,761
Other | Banking Subsidiaries
Debt Instrument [Line Items]
Long-term borrowed funds		$ 9	$ 16

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

[2] Reclassified to short-term borrowed funds.

[3] Rate disclosed reflects the floating rate as of December 31, 2018.

[4] Redeemed on June 29, 2018.