CONSOLIDATED STATEMENTS OF FINANCIAL CONDITION (Parenthetical) - USD ($) $ in Millions	Sep. 30, 2021	Sep. 30, 2020
Statement of Financial Position [Abstract]
Assets segregated pursuant to regulations, fair value	$ 2,100	$ 0
Trading assets, pledged as collateral	326	265
Available-for-sale securities, pledged as collateral	20	23
Other investments, pledged as collateral	$ 22	$ 37
Equity [Abstract]
Preferred stock, par value (in usd per share)	$ 0.10	$ 0.10
Preferred stock, authorized (in shares)	10,000,000	10,000,000
Preferred stock, issued (in shares)	0	0
Preferred stock, outstanding (in shares)	0	0
Common stock, par value (in usd per share)	$ 0.01	$ 0.01
Common stock, authorized (in shares)	350,000,000	350,000,000
Common stock, issued (in shares)	239,062,254	238,510,737
Common stock, outstanding (in shares)	205,738,821	204,834,839
Treasury stock (in shares)	33,323,433	33,675,898