Offsetting Assets and Liabilities - Additional Information (Details) - USD ($) $ in Millions	Dec. 31, 2019	Sep. 30, 2019
Offsetting Assets And Liabilities [Line Items]
Fair value of collateral the Company has received under enforceable master agreements	$ 1,780	$ 2,430
Fair value of collateral the Company has pledged under enforceable master agreements	2,190	2,860
Transacted Through the Option Clearing Corporation [Member]
Offsetting Assets And Liabilities [Line Items]
Gross amounts of deposits paid for securities borrowed	484	723
Gross amounts of deposits received for securities loaned	$ 1,690	$ 2,480