SECURITIES - Amortized Cost and Fair Value of Securities Available for Sale (Details) - USD ($) $ in Millions	Sep. 30, 2023	Dec. 31, 2022
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 3,520	$ 3,622
Gross Unrealized Gains	1	1
Gross Unrealized Losses	(376)	(348)
Fair Value	3,145	3,275
U.S. Treasury
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	327	278
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(21)	(21)
Fair Value	306	257
U.S. government agencies
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	85	107
Gross Unrealized Gains	1	1
Gross Unrealized Losses	0	0
Fair Value	86	108
U.S. government-sponsored entities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	323	283
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(20)	(21)
Fair Value	303	262
Agency mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,171	1,360
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(139)	(128)
Fair Value	1,032	1,232
Agency collateralized mortgage obligations
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	986	1,110
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(146)	(138)
Fair Value	840	972
Commercial mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	565	430
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(44)	(35)
Fair Value	521	395
States of the U.S. and political subdivisions (municipals)
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	30	33
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(4)	(4)
Fair Value	26	29
Other debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	33	21
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(2)	(1)
Fair Value	$ 31	$ 20

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amortized cost excluding accrued interest, before allowance for credit loss, of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount excluding accrued interest, of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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