Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents - Reconciliation of Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents (Details) - USD ($) $ in Millions	Dec. 31, 2019	Sep. 30, 2019	Dec. 31, 2018	Sep. 30, 2018
Cash and Cash Equivalents [Abstract]
Cash and cash equivalents	$ 2,551	$ 2,852
Restricted cash and restricted cash equivalents included in cash and investments segregated and on deposit for regulatory purposes	6,740	7,341
Total cash, cash equivalents, restricted cash and restricted cash equivalents shown in the Condensed Consolidated Statements of Cash Flows	$ 9,291	$ 10,193	$ 7,478	$ 4,548