AVAILABLE-FOR-SALE SECURITIES, Gross Unrealized Losses and Fair Value and Significant Assumptions (Details) - Bank - USD ($) $ in Millions	Mar. 31, 2024	Sep. 30, 2023
Debt Securities, Available-for-sale [Line Items]
Fair value less than 12 months	$ 187	$ 373
Unrealized losses less than 12 months	(1)	(5)
Fair value 12 months or more	8,274	8,617
Unrealized losses 12 months or more	(930)	(1,247)
Fair value	8,461	8,990
Unrealized losses	(931)	(1,252)
Agency residential MBS
Debt Securities, Available-for-sale [Line Items]
Fair value less than 12 months	50	73
Unrealized losses less than 12 months	(1)	(3)
Fair value 12 months or more	3,907	4,119
Unrealized losses 12 months or more	(475)	(651)
Fair value	3,957	4,192
Unrealized losses	(476)	(654)
Agency commercial MBS
Debt Securities, Available-for-sale [Line Items]
Fair value less than 12 months	0	3
Unrealized losses less than 12 months	0	0
Fair value 12 months or more	1,282	1,250
Unrealized losses 12 months or more	(165)	(211)
Fair value	1,282	1,253
Unrealized losses	(165)	(211)
Agency CMOs
Debt Securities, Available-for-sale [Line Items]
Fair value less than 12 months	10	0
Unrealized losses less than 12 months	0	0
Fair value 12 months or more	1,155	1,183
Unrealized losses 12 months or more	(216)	(265)
Fair value	1,165	1,183
Unrealized losses	(216)	(265)
Other agency obligations
Debt Securities, Available-for-sale [Line Items]
Fair value less than 12 months	0	97
Unrealized losses less than 12 months	0	(1)
Fair value 12 months or more	641	582
Unrealized losses 12 months or more	(17)	(30)
Fair value	641	679
Unrealized losses	(17)	(31)
Non-agency residential MBS
Debt Securities, Available-for-sale [Line Items]
Fair value less than 12 months	32	62
Unrealized losses less than 12 months	0	(1)
Fair value 12 months or more	433	401
Unrealized losses 12 months or more	(44)	(63)
Fair value	465	463
Unrealized losses	(44)	(64)
U.S. Treasuries
Debt Securities, Available-for-sale [Line Items]
Fair value less than 12 months	95	120
Unrealized losses less than 12 months	0	0
Fair value 12 months or more	760	995
Unrealized losses 12 months or more	(8)	(21)
Fair value	855	1,115
Unrealized losses	(8)	(21)
Corporate bonds
Debt Securities, Available-for-sale [Line Items]
Fair value less than 12 months	0	13
Unrealized losses less than 12 months	0	0
Fair value 12 months or more	87	78
Unrealized losses 12 months or more	(5)	(6)
Fair value	87	91
Unrealized losses	(5)	(6)
Other
Debt Securities, Available-for-sale [Line Items]
Fair value less than 12 months	0	5
Unrealized losses less than 12 months	0	0
Fair value 12 months or more	9	9
Unrealized losses 12 months or more	0	0
Fair value	9	14
Unrealized losses	$ 0	$ 0