Commitments and Contingencies - Summary of Cash Deposited with and Securities Pledged to Clearinghouses (Details) - USD ($) $ in Millions	Dec. 31, 2019	Sep. 30, 2019
Cash Deposited with and Securities Pledged to Clearinghouses [Line Items]
Cash deposited and securities pledged	$ 611	$ 713
Cash [Member] | Receivable from brokers, dealers and clearing organizations [Member]
Cash Deposited with and Securities Pledged to Clearinghouses [Line Items]
Cash deposited and securities pledged	447	545
U.S. government debt securities [Member] | Securities owned, at fair value [Member]
Cash Deposited with and Securities Pledged to Clearinghouses [Line Items]
Cash deposited and securities pledged	$ 164	$ 168