INVESTMENT SECURITIES AND OTHER SECURITIES - Continuous Unrealized Loss Position (Details) - USD ($) $ in Millions	Sep. 30, 2025	Dec. 31, 2024
Available-for-sale securities:
Less than 12 Months, Fair Value	$ 1,222	$ 4,726
Less than 12 Months, Gross Unrealized Losses	(13)	(74)
Over 12 Months, Fair Value	17,370	18,038
Over 12 Months, Gross Unrealized Losses	(2,562)	(3,409)
Total, Fair Value	18,592	22,764
Total, Gross Unrealized Losses	(2,575)	(3,483)
Held-to-maturity securities:
Less than 12 Months, Fair Value	189	1,843
Less than 12 Months, Gross Unrealized Losses	(1)	(25)
Over 12 Months, Fair Value	11,371	11,495
Over 12 Months, Gross Unrealized Losses	(1,747)	(2,260)
Total held-to-maturity securities	11,560	13,338
Total, Gross Unrealized Losses	(1,748)	(2,285)
U.S. Treasury
Available-for-sale securities:
Less than 12 Months, Fair Value	98	3,153
Less than 12 Months, Gross Unrealized Losses	(1)	(43)
Over 12 Months, Fair Value	798	0
Over 12 Months, Gross Unrealized Losses	(2)	0
Total, Fair Value	896	3,153
Total, Gross Unrealized Losses	(3)	(43)
Held-to-maturity securities:
Less than 12 Months, Fair Value	94	1,581
Less than 12 Months, Gross Unrealized Losses	0	(22)
Over 12 Months, Fair Value	362	0
Over 12 Months, Gross Unrealized Losses	(1)	0
Total held-to-maturity securities	456	1,581
Total, Gross Unrealized Losses	(1)	(22)
Total U.S. Treasury, federal agency, and other agency securities
Available-for-sale securities:
Less than 12 Months, Fair Value	360	3,692
Less than 12 Months, Gross Unrealized Losses	(1)	(49)
Over 12 Months, Fair Value	14,693	14,299
Over 12 Months, Gross Unrealized Losses	(2,453)	(3,171)
Total, Fair Value	15,053	17,991
Total, Gross Unrealized Losses	(2,454)	(3,220)
Held-to-maturity securities:
Less than 12 Months, Fair Value	189	1,843
Less than 12 Months, Gross Unrealized Losses	(1)	(25)
Over 12 Months, Fair Value	11,370	11,494
Over 12 Months, Gross Unrealized Losses	(1,747)	(2,260)
Total held-to-maturity securities	11,559	13,337
Total, Gross Unrealized Losses	(1,748)	(2,285)
Residential MBS
Available-for-sale securities:
Less than 12 Months, Fair Value	15	275
Less than 12 Months, Gross Unrealized Losses	0	(5)
Over 12 Months, Fair Value	9,356	9,676
Over 12 Months, Gross Unrealized Losses	(1,461)	(1,966)
Total, Fair Value	9,371	9,951
Total, Gross Unrealized Losses	(1,461)	(1,971)
Held-to-maturity securities:
Less than 12 Months, Fair Value	45	99
Less than 12 Months, Gross Unrealized Losses	(1)	(2)
Over 12 Months, Fair Value	6,822	7,097
Over 12 Months, Gross Unrealized Losses	(1,004)	(1,334)
Total held-to-maturity securities	6,867	7,196
Total, Gross Unrealized Losses	(1,005)	(1,336)
Residential CMO
Available-for-sale securities:
Less than 12 Months, Fair Value	218	243
Less than 12 Months, Gross Unrealized Losses	0	(1)
Over 12 Months, Fair Value	2,692	2,802
Over 12 Months, Gross Unrealized Losses	(324)	(433)
Total, Fair Value	2,910	3,045
Total, Gross Unrealized Losses	(324)	(434)
Held-to-maturity securities:
Less than 12 Months, Fair Value	50	163
Less than 12 Months, Gross Unrealized Losses	0	(1)
Over 12 Months, Fair Value	3,019	3,152
Over 12 Months, Gross Unrealized Losses	(543)	(690)
Total held-to-maturity securities	3,069	3,315
Total, Gross Unrealized Losses	(543)	(691)
Commercial MBS
Available-for-sale securities:
Less than 12 Months, Fair Value	0	0
Less than 12 Months, Gross Unrealized Losses	0	0
Over 12 Months, Fair Value	1,782	1,752
Over 12 Months, Gross Unrealized Losses	(663)	(767)
Total, Fair Value	1,782	1,752
Total, Gross Unrealized Losses	(663)	(767)
Held-to-maturity securities:
Less than 12 Months, Fair Value	0	0
Less than 12 Months, Gross Unrealized Losses	0	0
Over 12 Months, Fair Value	1,117	1,176
Over 12 Months, Gross Unrealized Losses	(196)	(231)
Total held-to-maturity securities	1,117	1,176
Total, Gross Unrealized Losses	(196)	(231)
Other agencies
Available-for-sale securities:
Less than 12 Months, Fair Value	29	21
Less than 12 Months, Gross Unrealized Losses	0	0
Over 12 Months, Fair Value	65	69
Over 12 Months, Gross Unrealized Losses	(3)	(5)
Total, Fair Value	94	90
Total, Gross Unrealized Losses	(3)	(5)
Held-to-maturity securities:
Less than 12 Months, Fair Value	0	0
Less than 12 Months, Gross Unrealized Losses	0	0
Over 12 Months, Fair Value	50	69
Over 12 Months, Gross Unrealized Losses	(3)	(5)
Total held-to-maturity securities	50	69
Total, Gross Unrealized Losses	(3)	(5)
Municipal securities
Available-for-sale securities:
Less than 12 Months, Fair Value	862	985
Less than 12 Months, Gross Unrealized Losses	(12)	(25)
Over 12 Months, Fair Value	2,167	2,336
Over 12 Months, Gross Unrealized Losses	(75)	(107)
Total, Fair Value	3,029	3,321
Total, Gross Unrealized Losses	(87)	(132)
Held-to-maturity securities:
Less than 12 Months, Fair Value	0	0
Less than 12 Months, Gross Unrealized Losses	0	0
Over 12 Months, Fair Value	1	1
Over 12 Months, Gross Unrealized Losses	0	0
Total held-to-maturity securities	1	1
Total, Gross Unrealized Losses	0	0
Corporate debt
Available-for-sale securities:
Less than 12 Months, Fair Value	0	0
Less than 12 Months, Gross Unrealized Losses	0	0
Over 12 Months, Fair Value	175	1,053
Over 12 Months, Gross Unrealized Losses	(17)	(102)
Total, Fair Value	175	1,053
Total, Gross Unrealized Losses	(17)	(102)
Asset-backed securities
Available-for-sale securities:
Less than 12 Months, Fair Value	0	49
Less than 12 Months, Gross Unrealized Losses	0	0
Over 12 Months, Fair Value	254	263
Over 12 Months, Gross Unrealized Losses	(10)	(19)
Total, Fair Value	254	312
Total, Gross Unrealized Losses	(10)	(19)
Private-label CMO
Available-for-sale securities:
Less than 12 Months, Fair Value	0	0
Less than 12 Months, Gross Unrealized Losses	0	0
Over 12 Months, Fair Value	81	87
Over 12 Months, Gross Unrealized Losses	(7)	(10)
Total, Fair Value	81	87
Total, Gross Unrealized Losses	$ (7)	$ (10)