INVESTMENT SECURITIES AND OTHER SECURITIES - Schedule of Amortized Cost, Fair Value, and Gross Unrealized Gains and Losses (Details) - USD ($) $ in Millions	Sep. 30, 2025	Dec. 31, 2024
Available-for-sale securities:
Total available-for-sale securities	$ 28,602	$ 30,743
Available-for-sale securities, Gross Gains	58	13
Available-for-sale securities, Gross Losses	(2,575)	(3,483)
Available-for-sale securities	26,085	27,273
Held-to-maturity securities:
Held-to-maturity securities	15,597	16,368
Held-to-maturity securities, Gross Gains	20	3
Held-to-maturity securities, Gross Losses	(1,748)	(2,285)
Held-to-maturity securities, Fair Value	13,869	14,086
Other securities, at cost:
Other securities, Amortized Cost	870	822
Other securities, Gross Gains	0	1
Other securities, Gross Losses	0	0
Other securities, Fair Value	870	823
Accrued interest receivable on AFS securities	89	89
Accrued interest receivable on HTM securities	41	46
Cumulative Amount of Fair Value Hedging Adjustment To Hedged Items	160	458
U.S. Treasury
Available-for-sale securities:
Total available-for-sale securities	5,371	6,588
Available-for-sale securities, Gross Gains	42	11
Available-for-sale securities, Gross Losses	(3)	(43)
Available-for-sale securities	5,410	6,556
Held-to-maturity securities:
Held-to-maturity securities	2,339	2,045
Held-to-maturity securities, Gross Gains	16	0
Held-to-maturity securities, Gross Losses	(1)	(22)
Held-to-maturity securities, Fair Value	2,354	2,023
US Treasury and Government
Available-for-sale securities:
Total available-for-sale securities	23,826	25,008
Available-for-sale securities, Gross Gains	50	12
Available-for-sale securities, Gross Losses	(2,454)	(3,220)
Available-for-sale securities	21,422	21,800
Held-to-maturity securities:
Held-to-maturity securities	15,596	16,367
Held-to-maturity securities, Gross Gains	20	3
Held-to-maturity securities, Gross Losses	(1,748)	(2,285)
Held-to-maturity securities, Fair Value	13,868	14,085
Residential MBS
Available-for-sale securities:
Total available-for-sale securities	11,133	11,988
Available-for-sale securities, Gross Gains	3	0
Available-for-sale securities, Gross Losses	(1,461)	(1,971)
Available-for-sale securities	9,675	10,017
Held-to-maturity securities:
Held-to-maturity securities	7,919	8,533
Held-to-maturity securities, Gross Gains	0	0
Held-to-maturity securities, Gross Losses	(1,005)	(1,336)
Held-to-maturity securities, Fair Value	6,914	7,197
Residential CMO
Available-for-sale securities:
Total available-for-sale securities	4,761	3,778
Available-for-sale securities, Gross Gains	5	1
Available-for-sale securities, Gross Losses	(324)	(434)
Available-for-sale securities	4,442	3,345
Held-to-maturity securities:
Held-to-maturity securities	3,971	4,309
Held-to-maturity securities, Gross Gains	4	3
Held-to-maturity securities, Gross Losses	(543)	(691)
Held-to-maturity securities, Fair Value	3,432	3,621
Commercial MBS
Available-for-sale securities:
Total available-for-sale securities	2,445	2,519
Available-for-sale securities, Gross Gains	0	0
Available-for-sale securities, Gross Losses	(663)	(767)
Available-for-sale securities	1,782	1,752
Held-to-maturity securities:
Held-to-maturity securities	1,314	1,407
Held-to-maturity securities, Gross Gains	0	0
Held-to-maturity securities, Gross Losses	(196)	(231)
Held-to-maturity securities, Fair Value	1,118	1,176
Other agencies
Available-for-sale securities:
Total available-for-sale securities	116	135
Available-for-sale securities, Gross Gains	0	0
Available-for-sale securities, Gross Losses	(3)	(5)
Available-for-sale securities	113	130
Held-to-maturity securities:
Held-to-maturity securities	53	73
Held-to-maturity securities, Gross Gains	0	0
Held-to-maturity securities, Gross Losses	(3)	(5)
Held-to-maturity securities, Fair Value	50	68
Municipal securities
Available-for-sale securities:
Total available-for-sale securities	4,194	4,119
Available-for-sale securities, Gross Gains	8	1
Available-for-sale securities, Gross Losses	(87)	(132)
Available-for-sale securities	4,115	3,988
Held-to-maturity securities:
Held-to-maturity securities	1	1
Held-to-maturity securities, Gross Gains	0	0
Held-to-maturity securities, Gross Losses	0	0
Held-to-maturity securities, Fair Value	1	1
Corporate debt
Available-for-sale securities:
Total available-for-sale securities	192	119
Available-for-sale securities, Gross Gains	0	0
Available-for-sale securities, Gross Losses	(17)	(10)
Available-for-sale securities	175	109
Asset-backed securities
Available-for-sale securities:
Total available-for-sale securities	269	330
Available-for-sale securities, Gross Gains	0	0
Available-for-sale securities, Gross Losses	(10)	(19)
Available-for-sale securities	259	311
Private-label CMO
Available-for-sale securities:
Total available-for-sale securities	111	1,157
Available-for-sale securities, Gross Gains	0	0
Available-for-sale securities, Gross Losses	(7)	(102)
Available-for-sale securities	104	1,055
Other securities/sovereign debt
Available-for-sale securities:
Total available-for-sale securities	10	10
Available-for-sale securities, Gross Gains	0	0
Available-for-sale securities, Gross Losses	0	0
Available-for-sale securities	10	10
Non-marketable equity securities: | FRB stock
Other securities, at cost:
Other securities, Amortized Cost	575	521
Other securities, Gross Gains	0	0
Other securities, Gross Losses	0	0
Other securities, Fair Value	575	521
Non-marketable equity securities: | FHLB stock
Other securities, at cost:
Other securities, Amortized Cost	237	246
Other securities, Gross Gains	0	0
Other securities, Gross Losses	0	0
Other securities, Fair Value	237	246
Other non-marketable equity securities
Other securities, at cost:
Other securities, Amortized Cost	26	25
Other securities, Gross Gains	0	0
Other securities, Gross Losses	0	0
Other securities, Fair Value	26	25
Mutual funds
Other securities, at cost:
Other securities, Amortized Cost	30	29
Other securities, Gross Gains	0	0
Other securities, Gross Losses	0	0
Other securities, Fair Value	30	29
Equity securities
Other securities, at cost:
Other securities, Amortized Cost	2	1
Other securities, Gross Gains	0	1
Other securities, Gross Losses	0	0
Other securities, Fair Value	$ 2	$ 2