BORROWED FUNDS - Long Term Debt (Details) - USD ($) $ in Millions	Jun. 30, 2020	Dec. 31, 2019
Debt Instrument [Line Items]
Long-term borrowed funds	$ 9,202	$ 14,047
Parent Company
Debt Instrument [Line Items]
Long-term borrowed funds	3,528
Parent Company \| Subordinated Debt \| 4.150% fixed-rate subordinated debt, due September 2022
Debt Instrument [Line Items]
Long-term borrowed funds	$ 349	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	750
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	$ 350	349
Interest rate	2.375%
Parent Company \| Senior Unsecured Notes \| 2.850% fixed-rate senior unsecured notes, due July 2026
Debt Instrument [Line Items]
Long-term borrowed funds	$ 497	496
Interest rate	2.85%
Parent Company \| Senior Unsecured Notes \| 2.500% fixed-rate senior unsecured notes, due February 2030
Debt Instrument [Line Items]
Long-term borrowed funds	$ 297	0
Interest rate	2.50%
Parent Company \| Senior Unsecured Notes \| 3.250% fixed-rate senior unsecured notes, due April 2030
Debt Instrument [Line Items]
Long-term borrowed funds	$ 744	0
Interest rate	3.25%
CBNA and Other Subsidiaries
Debt Instrument [Line Items]
Long-term borrowed funds	$ 5,674
CBNA and Other Subsidiaries \| Senior Unsecured Notes \| 2.250% senior unsecured notes, due March 2020
Debt Instrument [Line Items]
Long-term borrowed funds	$ 0	700
Interest rate	2.25%
CBNA and Other Subsidiaries \| Senior Unsecured Notes \| 2.678% floating-rate senior unsecured notes, due 2020
Debt Instrument [Line Items]
Long-term borrowed funds	$ 0	300
Interest rate	2.678%
CBNA and Other Subsidiaries \| Senior Unsecured Notes \| 2.217% floating-rate senior unsecured notes, due 2020
Debt Instrument [Line Items]
Long-term borrowed funds	$ 0	250
Interest rate	2.217%
CBNA and Other Subsidiaries \| Senior Unsecured Notes \| 2.200% senior unsecured notes, due May 2020
Debt Instrument [Line Items]
Long-term borrowed funds	$ 0	500
Interest rate	2.20%
CBNA and Other Subsidiaries \| Senior Unsecured Notes \| 2.250% senior unsecured notes, due October 2020
Debt Instrument [Line Items]
Long-term borrowed funds	$ 753	750
Interest rate	2.25%
CBNA and Other Subsidiaries \| Senior Unsecured Notes \| 2.550% senior unsecured notes, due May 2021
Debt Instrument [Line Items]
Long-term borrowed funds	$ 1,006	991
Interest rate	2.55%
CBNA and Other Subsidiaries \| Senior Unsecured Notes \| 3.250% senior unsecured notes, due February 2022
Debt Instrument [Line Items]
Long-term borrowed funds	$ 723	711
Interest rate	3.25%
CBNA and Other Subsidiaries \| Senior Unsecured Notes \| 1.144% Floating Rate Senior Unsecured Notes, Due February 2022
Debt Instrument [Line Items]
Long-term borrowed funds	$ 299	299
Interest rate	1.144%
CBNA and Other Subsidiaries \| Senior Unsecured Notes \| 1.170% floating-rate senior unsecured notes, due May 2022
Debt Instrument [Line Items]
Long-term borrowed funds	$ 250	250
Interest rate	1.17%
CBNA and Other Subsidiaries \| Senior Unsecured Notes \| 2.650% senior unsecured notes, due May 2022
Debt Instrument [Line Items]
Long-term borrowed funds	$ 514	501
Interest rate	2.65%
CBNA and Other Subsidiaries \| Senior Unsecured Notes \| 3.700% senior unsecured notes, due March 2023
Debt Instrument [Line Items]
Long-term borrowed funds	$ 533	515
Interest rate	3.70%
CBNA and Other Subsidiaries \| Senior Unsecured Notes \| 1.256% floating-rate senior unsecured notes, due March 2023
Debt Instrument [Line Items]
Long-term borrowed funds	$ 249	249
Interest rate	1.256%
CBNA and Other Subsidiaries \| Senior Unsecured Notes \| 2.250% senior unsecured notes, due April 2025
Debt Instrument [Line Items]
Long-term borrowed funds	$ 746	0
Interest rate	2.25%
CBNA and Other Subsidiaries \| Senior Unsecured Notes \| 3.750% senior unsecured notes, due February 2026
Debt Instrument [Line Items]
Long-term borrowed funds	$ 558	521
Interest rate	3.75%
CBNA and Other Subsidiaries \| Federal Home Loan Bank advances
Debt Instrument [Line Items]
Long-term borrowed funds	$ 6	5,008
Weighted average rate	1.599%
CBNA and Other Subsidiaries \| Other
Debt Instrument [Line Items]
Long-term borrowed funds	$ 37	$ 18

Contractual interest rate for funds borrowed, under the debt agreement.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22(a)(1))
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Weighted average interest rate on advances made and reported by Federal Home Loan Bank (FHLBank).

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Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 20
-Section 50
-Paragraph 4
-Subparagraph (b)(3)
-URI http://asc.fasb.org/extlink&oid=109500613&loc=SL6031897-161870

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(16))
-URI http://asc.fasb.org/extlink&oid=119400593&loc=d3e572229-122910

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
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Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
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-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
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-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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