Offsetting Assets and Liabilities - Disaggregation of Secured Lending Transactions (Details) - USD ($) $ in Millions	Dec. 31, 2019	Sep. 30, 2019
Disaggregation of Gross Securities Lending Transactions [Line Items]
Deposits received for securities loaned, gross	$ 2,394	$ 3,189
Equity Securities [Member]
Disaggregation of Gross Securities Lending Transactions [Line Items]
Deposits received for securities loaned, gross	1,922	2,629
Exchange-traded Funds [Member]
Disaggregation of Gross Securities Lending Transactions [Line Items]
Deposits received for securities loaned, gross	300	285
Real Estate Investment Trusts [Member]
Disaggregation of Gross Securities Lending Transactions [Line Items]
Deposits received for securities loaned, gross	85	181
Closed-end Funds [Member]
Disaggregation of Gross Securities Lending Transactions [Line Items]
Deposits received for securities loaned, gross	76	86
Other [Member]
Disaggregation of Gross Securities Lending Transactions [Line Items]
Deposits received for securities loaned, gross	$ 11	$ 8