FAIR VALUE MEASUREMENTS - Assets and Liabilities on Recurring Basis (Details) - USD ($) $ in Millions	Sep. 30, 2023	Dec. 31, 2022
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities available for sale	$ 3,145	$ 3,275
Loans held for sale	92	91	[1]
Derivative financial instruments	145	96
Derivative financial instruments	469	411
U.S. Treasury
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities available for sale	306	257
U.S. government agencies
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities available for sale	86	108
U.S. government-sponsored entities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities available for sale	303	262
Agency mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities available for sale	1,032	1,232
Agency collateralized mortgage obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities available for sale	840	972
Commercial mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities available for sale	521	395
States of the U.S. and political subdivisions (municipals)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities available for sale	26	29
Other debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities available for sale	31	20
Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities available for sale	306	257
Derivative financial instruments	0	0
Derivative financial instruments	0	0
Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities available for sale	2,839	3,018
Derivative financial instruments	145	96
Derivative financial instruments	466	399
Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities available for sale	0	0
Derivative financial instruments	0	0
Derivative financial instruments	3	12
Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities available for sale	3,145	3,275
Loans held for sale	92	91
Derivative financial instruments	145	96
Total assets measured at fair value on a recurring basis	3,382	3,462
Derivative financial instruments	469	411
Total liabilities measured at fair value on a recurring basis	469	411
Fair Value, Recurring | U.S. Treasury
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities available for sale	306	257
Fair Value, Recurring | U.S. government agencies
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities available for sale	86	108
Fair Value, Recurring | U.S. government-sponsored entities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities available for sale	303	262
Fair Value, Recurring | Agency mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities available for sale	1,032	1,232
Fair Value, Recurring | Agency collateralized mortgage obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities available for sale	840	972
Fair Value, Recurring | Commercial mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities available for sale	521	395
Fair Value, Recurring | States of the U.S. and political subdivisions (municipals)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities available for sale	26	29
Fair Value, Recurring | Other debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities available for sale	31	20
Fair Value, Recurring | Trading
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative financial instruments	462	396
Fair Value, Recurring | Not for trading
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative financial instruments	7	15
Fair Value, Recurring | Trading
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative financial instruments	142	95
Fair Value, Recurring | Not for trading
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative financial instruments	3	1
Fair Value, Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities available for sale	306	257
Loans held for sale	0	0
Derivative financial instruments	0	0
Total assets measured at fair value on a recurring basis	306	257
Derivative financial instruments	0	0
Total liabilities measured at fair value on a recurring basis	0	0
Fair Value, Recurring | Level 1 | U.S. Treasury
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities available for sale	306	257
Fair Value, Recurring | Level 1 | U.S. government agencies
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities available for sale	0	0
Fair Value, Recurring | Level 1 | U.S. government-sponsored entities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities available for sale	0	0
Fair Value, Recurring | Level 1 | Agency mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities available for sale	0	0
Fair Value, Recurring | Level 1 | Agency collateralized mortgage obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities available for sale	0	0
Fair Value, Recurring | Level 1 | Commercial mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities available for sale	0	0
Fair Value, Recurring | Level 1 | States of the U.S. and political subdivisions (municipals)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities available for sale	0	0
Fair Value, Recurring | Level 1 | Other debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities available for sale	0	0
Fair Value, Recurring | Level 1 | Trading
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative financial instruments	0	0
Fair Value, Recurring | Level 1 | Not for trading
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative financial instruments	0	0
Fair Value, Recurring | Level 1 | Trading
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative financial instruments	0	0
Fair Value, Recurring | Level 1 | Not for trading
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative financial instruments	0	0
Fair Value, Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities available for sale	2,839	3,018
Loans held for sale	92	91
Derivative financial instruments	145	96
Total assets measured at fair value on a recurring basis	3,076	3,205
Derivative financial instruments	466	399
Total liabilities measured at fair value on a recurring basis	466	399
Fair Value, Recurring | Level 2 | U.S. Treasury
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities available for sale	0	0
Fair Value, Recurring | Level 2 | U.S. government agencies
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities available for sale	86	108
Fair Value, Recurring | Level 2 | U.S. government-sponsored entities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities available for sale	303	262
Fair Value, Recurring | Level 2 | Agency mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities available for sale	1,032	1,232
Fair Value, Recurring | Level 2 | Agency collateralized mortgage obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities available for sale	840	972
Fair Value, Recurring | Level 2 | Commercial mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities available for sale	521	395
Fair Value, Recurring | Level 2 | States of the U.S. and political subdivisions (municipals)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities available for sale	26	29
Fair Value, Recurring | Level 2 | Other debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities available for sale	31	20
Fair Value, Recurring | Level 2 | Trading
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative financial instruments	462	396
Fair Value, Recurring | Level 2 | Not for trading
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative financial instruments	4	3
Fair Value, Recurring | Level 2 | Trading
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative financial instruments	142	95
Fair Value, Recurring | Level 2 | Not for trading
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative financial instruments	3	1
Fair Value, Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities available for sale	0	0
Loans held for sale	0	0
Derivative financial instruments	0	0
Total assets measured at fair value on a recurring basis	0	0
Derivative financial instruments	3	12
Total liabilities measured at fair value on a recurring basis	3	12
Fair Value, Recurring | Level 3 | U.S. Treasury
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities available for sale	0	0
Fair Value, Recurring | Level 3 | U.S. government agencies
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities available for sale	0	0
Fair Value, Recurring | Level 3 | U.S. government-sponsored entities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities available for sale	0	0
Fair Value, Recurring | Level 3 | Agency mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities available for sale	0	0
Fair Value, Recurring | Level 3 | Agency collateralized mortgage obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities available for sale	0	0
Fair Value, Recurring | Level 3 | Commercial mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities available for sale	0	0
Fair Value, Recurring | Level 3 | States of the U.S. and political subdivisions (municipals)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities available for sale	0	0
Fair Value, Recurring | Level 3 | Other debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities available for sale	0	0
Fair Value, Recurring | Level 3 | Trading
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative financial instruments	0	0
Fair Value, Recurring | Level 3 | Not for trading
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative financial instruments	3	12
Fair Value, Recurring | Level 3 | Trading
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative financial instruments	0	0
Fair Value, Recurring | Level 3 | Not for trading
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative financial instruments	$ 0	$ 0

[1] Amount represents loans for which we have elected the fair value option. See Note 19.