SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES, Other Narrative (Details) $ in Millions	12 Months Ended
	Sep. 30, 2021 USD ($) portfolio_segment tax_credit_fund investor institution subsidiary	Sep. 30, 2020 USD ($)
Assets segregated for regulatory purposes and restricted cash
Assets segregated for regulatory purposes and restricted cash	$ 11,348	$ 4,244
Bank loans and allowances for losses [Abstract]
Number of loan portfolio segments | portfolio_segment	6
Minimum past due for loans placed on nonaccrual status (or more)	90 days
Period of satisfactory performance for loans to be returned to accrual status	6 months
Minimum amount of Shared National Credit (SNC) loan syndications	$ 100
Minimum number of regulated institutions with which SNCs are shared (or more) | institution	3
Minimum past due for residential loans to be reviewed	60 days
Minimum past due for charge-offs to be considered on residential mortgage loans	90 days
Maximum past due for charge-offs taken on residential mortgage loans	120 days
Minimum period for which updated valuation is obtained for loans in pre-foreclosure status	9 months
Brokerage client receivables and loans to financial advisors [Abstract]
Repayment period of loans to financial advisors and certain key revenue producers, minimum	5 years
Repayment period of loans to financial advisors and certain key revenue producers, maximum	10 years
LIHTC Funds [Abstract]
Minimum number of investor members or limited partners of LIHTC Funds (or more) | investor	1
Number of general types of non-guaranteed tax credit funds | tax_credit_fund	2
Number of single investor members or limited partners in funds | subsidiary	1
Number of party with power | subsidiary	1
US Treasury Securities
Assets segregated for regulatory purposes and restricted cash
Assets segregated for regulatory purposes and restricted cash	$ 3,550
US Treasury Securities | US Treasury Securities
Assets segregated for regulatory purposes and restricted cash
Assets segregated for regulatory purposes and restricted cash	$ 2,100