CONDENSED CONSOLIDATED STATEMENTS OF FINANCIAL CONDITION (Unaudited) (Parenthetical) - USD ($) $ in Millions	Jun. 30, 2020	Sep. 30, 2019
Statement of Financial Position [Abstract]
Financial instruments, owned and pledged as collateral, at fair value	$ 270	$ 535
Available-for-sale securities, pledged as collateral	24	24
Other investments, owned and pledged as collateral, at fair value	$ 40	$ 32
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
Preferred stock, shares outstanding (in shares)	0
Common stock, par value per share (in usd per share)	$ 0.01	$ 0.01
Common stock, shares authorized (in shares)	350,000,000	350,000,000
Common stock, shares issued (in shares)	158,909,194	158,435,030
Common stock, shares outstanding (in shares)	137,025,724	137,841,952
Treasury stock (in shares)	21,883,470	20,593,078