Cash and Investments Segregated and on Deposit for Regulatory Purposes (Details) - USD ($) $ in Millions	Dec. 31, 2019	Sep. 30, 2019
Restricted Cash and Cash Equivalents Items [Line Items]
Cash and investments segregated and on deposit for regulatory purposes	$ 8,536	$ 8,684
U.S. government debt securities [Member]
Restricted Cash and Cash Equivalents Items [Line Items]
Cash and investments segregated and on deposit for regulatory purposes	3,747	4,369
Cash in demand deposit accounts [Member]
Restricted Cash and Cash Equivalents Items [Line Items]
Cash and investments segregated and on deposit for regulatory purposes	2,300	2,304
U.S. government agency mortgage-backed securities [Member]
Restricted Cash and Cash Equivalents Items [Line Items]
Cash and investments segregated and on deposit for regulatory purposes	1,771	1,318
Reverse repurchase agreements (collateralized by U.S. government debt securities) [Member]
Restricted Cash and Cash Equivalents Items [Line Items]
Cash and investments segregated and on deposit for regulatory purposes	500	500
Cash on deposit with futures commission merchants [Member]
Restricted Cash and Cash Equivalents Items [Line Items]
Cash and investments segregated and on deposit for regulatory purposes	193	168
U.S. government debt securities on deposit with futures commission merchant [Member]
Restricted Cash and Cash Equivalents Items [Line Items]
Cash and investments segregated and on deposit for regulatory purposes	$ 25	$ 25