SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES, Recent Accounting Developments (Details) - USD ($) $ in Millions	Sep. 30, 2021	Oct. 01, 2020	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Loans to financial advisors, allowance for credit loss	$ 27		$ 4
Funded bank loans, allowance for credit losses	320		354	$ 218	$ 203	$ 190
Retained earnings	(7,633)		(6,484)
Total loans held for investment
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Funded bank loans, allowance for credit losses	$ 320		354	$ 218
Cumulative adjustments for changes in accounting principles
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses		$ 42
Loans to financial advisors, allowance for credit loss		25
Retained earnings		35
Cumulative adjustments for changes in accounting principles | Total loans held for investment
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Funded bank loans, allowance for credit losses			$ 9
Cumulative adjustments for changes in accounting principles | Unfunded lending commitments
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Funded bank loans, allowance for credit losses		$ 8