ACQUISITIONS, All Other Acquisitions - Purchase Consideration, Fair Value Estimates of Assets Acquired and Liabilities Assumed, and Resulting Goodwill (Details) - USD ($) $ / shares in Units, $ in Millions		6 Months Ended	12 Months Ended
	Jun. 01, 2022	Jul. 01, 2022	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2022
Fair value of common stock issued for TriState Capital acquisition:
RSAs issued for TriState Capital acquisition			$ 0	$ 0	$ 28
Fair value of liabilities assumed:
Derivative liabilities		$ 125
Goodwill			1,451	1,437	1,422
PCG
Fair value of liabilities assumed:
Goodwill			578	564	550
Capital Markets
Fair value of liabilities assumed:
Goodwill			275	275	274
Common stock
Fair value of common stock issued for TriState Capital acquisition:
Total fair value of common stock issued for TriState Capital acquisition			0	0	778
Preferred stock
Fair value of common stock issued for TriState Capital acquisition:
Total fair value of common stock issued for TriState Capital acquisition			$ 0	$ 0	$ 120
Charles Stanley, TriState Capital, and SumRidge Partners
Fair value of common stock issued for TriState Capital acquisition:
Other common stock consideration	$ 10
Total fair value of common stock issued for TriState Capital acquisition	778
Effective settlement of the Note related to the TriState Capital acquisition	123
RSAs issued for TriState Capital acquisition	$ 28
Aggregate cash consideration paid for Charles Stanley, TriState Capital, and SumRidge Partners acquisitions		1,045
Total fair value of aggregate purchase consideration		2,094
Fair value of assets acquired:
Cash and cash equivalents		613
Assets segregated for regulatory purposes		1,890
Trading assets		631
Available-for-sale securities		1,524
Derivative assets		51
Brokerage client receivables		98
Other receivables		479
Bank loans		11,549
Identifiable intangible assets		334
All other assets acquired		303
Total assets acquired		17,472
Fair value of liabilities assumed:
Bank deposits		12,593
Trading liabilities		552
Brokerage client payables		2,064
Other borrowings		375
All other liabilities assumed		464
Total liabilities assumed		16,173
Fair value of net identifiable assets acquired		1,299
Goodwill		795
Charles Stanley, TriState Capital, and SumRidge Partners | PCG
Fair value of liabilities assumed:
Goodwill		164
Charles Stanley, TriState Capital, and SumRidge Partners | Capital Markets
Fair value of liabilities assumed:
Goodwill		102
Charles Stanley, TriState Capital, and SumRidge Partners | Bank
Fair value of liabilities assumed:
Goodwill		$ 529
Charles Stanley, TriState Capital, and SumRidge Partners | Common stock
Fair value of common stock issued for TriState Capital acquisition:
Shares issued (in shares)	7,861,189
Share price (in dollars per share)	$ 97.74
Stock issued	$ 768
Charles Stanley, TriState Capital, and SumRidge Partners | Preferred stock
Fair value of common stock issued for TriState Capital acquisition:
Total fair value of common stock issued for TriState Capital acquisition	120
TriState Capital
Fair value of common stock issued for TriState Capital acquisition:
Effective settlement of the Note related to the TriState Capital acquisition	123
RSAs issued for TriState Capital acquisition	$ 28
TriState Capital | TriState Capital Common Stock
Fair value of liabilities assumed:
Share price (in dollars per share)	$ 6.00
TriState Capital | TriState Series C Convertible Preferred Stock
Fair value of liabilities assumed:
Share price (in dollars per share)	$ 30