CONDENSED CONSOLIDATED STATEMENTS OF FINANCIAL CONDITION (Unaudited) (Parenthetical) - USD ($) $ in Millions	Mar. 31, 2022	Sep. 30, 2021
Statement of Financial Position [Abstract]
Assets segregated pursuant to regulations, fair value	$ 10,394	$ 2,100
Financial instruments, owned and pledged as collateral, at fair value	225	326
Available-for-sale securities, pledged as collateral	28	20
Other investments, owned and pledged as collateral, at fair value	$ 22	$ 22
Equity [Abstract]
Preferred stock, par value per share (in usd per share)	$ 0.10	$ 0.10
Preferred stock, shares authorized (in shares)	10,000,000	10,000,000
Preferred stock, shares issued (in shares)	0	0
Preferred stock, shares outstanding (in shares)	0	0
Common stock, par value per share (in usd per share)	$ 0.01	$ 0.01
Common stock, shares authorized (in shares)	650,000,000	350,000,000
Common stock, shares issued (in shares)	239,295,583	239,062,254
Common stock, shares outstanding (in shares)	207,897,379	205,738,821
Treasury stock (in shares)	31,398,204	33,323,433