VARIABLE INTEREST ENTITIES, Primary Beneficiary - Carrying Value of Assets, Liabilities and Equity (Details) - USD ($) $ in Millions	Mar. 31, 2021	Sep. 30, 2020	Mar. 31, 2020	Sep. 30, 2019
Assets:
Cash and cash equivalents and assets segregated pursuant to regulations	$ 10,275	$ 9,634	$ 14,693	$ 5,971
Other assets	1,616	1,525
Total assets	56,066	47,482
Liabilities:
Other payables	1,608	1,513
Total liabilities	48,429	40,306
Noncontrolling interests	45	62
Variable Interest Entity, Primary Beneficiary
Assets:
Total assets	137	221
Liabilities:
Total liabilities	31	94
Variable Interest Entity, Primary Beneficiary | Carrying amount
Assets:
Cash and cash equivalents and assets segregated pursuant to regulations	11	9
Other investments	46	37
Other assets	59	164
Total assets	116	210
Liabilities:
Other payables	3	76
Total liabilities	3	76
Noncontrolling interests	$ 42	$ 62