Settlement Guarantee Management - Collateral (Detail) - USD ($) $ in Millions	Mar. 31, 2021	Sep. 30, 2020
Settlement Guarantee Management [Abstract]
Restricted cash and restricted cash equivalents	$ 2,057	$ 1,850
Pledged securities at market value	279	228
Letters of credit	1,334	1,306
Guarantees	635	717
Total	$ 4,305	$ 4,101