CONDENSED CONSOLIDATED STATEMENTS OF FINANCIAL CONDITION (Unaudited) (Parenthetical) - USD ($) $ in Millions	Mar. 31, 2020	Sep. 30, 2019
Statement of Financial Position [Abstract]
Trading instruments, pledged as collateral	$ 236	$ 535
Available-for-sale securities, pledged as collateral	28	24
Other investments, pledged as collateral	$ 40	$ 32
Equity:
Preferred stock, par value (in dollars 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 (in dollars per share)	$ 0.01	$ 0.01
Common stock, shares authorized (in shares)	350,000,000	350,000,000
Common stock, shares issued (in shares)	158,731,812	158,435,030
Common stock, shares outstanding (in shares)	136,787,387	137,841,952
Treasury stock, shares (in shares)	21,944,425	20,593,078