INVESTMENT SECURITIES - Gross Unrealized Losses and Fair Values of Securities (Details) - USD ($) $ in Millions	Jun. 30, 2024	Dec. 31, 2023
Fair Value
Less than 12 months	$ 4,340	$ 3,621
12 months or more	11,207	11,046
Total	15,547	14,667
Gross Unrealized Losses
Less than 12 months	(66)	(28)
12 months or more	(1,419)	(1,300)
Total	(1,485)	(1,328)
U.S. Treasury and agency obligations
Fair Value
Less than 12 months	1,592	1,899
12 months or more	1,135	431
Total	2,727	2,330
Gross Unrealized Losses
Less than 12 months	(32)	(12)
12 months or more	(60)	(39)
Total	(92)	(51)
Mortgage-backed securities
Fair Value
Less than 12 months	734	958
12 months or more	2,732	2,269
Total	3,466	3,227
Gross Unrealized Losses
Less than 12 months	(14)	(12)
12 months or more	(495)	(413)
Total	(509)	(425)
Tax-exempt municipal securities
Fair Value
Less than 12 months	208	160
12 months or more	503	523
Total	711	683
Gross Unrealized Losses
Less than 12 months	(2)	(1)
12 months or more	(25)	(21)
Total	(27)	(22)
Residential
Fair Value
Less than 12 months	21	0
12 months or more	353	373
Total	374	373
Gross Unrealized Losses
Less than 12 months	(1)	0
12 months or more	(69)	(66)
Total	(70)	(66)
Commercial
Fair Value
Less than 12 months	54	18
12 months or more	1,179	1,303
Total	1,233	1,321
Gross Unrealized Losses
Less than 12 months	(1)	0
12 months or more	(111)	(126)
Total	(112)	(126)
Asset-backed securities
Fair Value
Less than 12 months	398	120
12 months or more	735	1,364
Total	1,133	1,484
Gross Unrealized Losses
Less than 12 months	(2)	(1)
12 months or more	(33)	(43)
Total	(35)	(44)
Corporate debt securities
Fair Value
Less than 12 months	1,333	466
12 months or more	4,570	4,783
Total	5,903	5,249
Gross Unrealized Losses
Less than 12 months	(14)	(2)
12 months or more	(626)	(592)
Total	$ (640)	$ (594)