Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents - Summary of Cash and Cash Equivalents (Details) - USD ($) $ in Millions	Dec. 31, 2019	Sep. 30, 2019
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents	$ 2,551	$ 2,852
Broker-dealer subsidiaries [Member]
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents	2,051	2,260
Corporate [Member]
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents	267	366
Futures commission merchant and forex dealer member subsidiary [Member]
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents	108	94
Trust company subsidiary [Member]
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents	98	124
Investment advisory subsidiaries [Member]
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents	$ 27	$ 8