Offsetting Assets and Liabilities - Effect of Rights of Setoff Associated with Company's Recognized Assets and Liabilities (Details) - USD ($) $ in Millions	Dec. 31, 2019	Sep. 30, 2019
Offsetting Derivative Asset, Securities Purchased under Agreements to Resell, Securities Borrowed [Abstract]
Total recognized assets, gross	$ 1,789	$ 2,435
Total recognized assets, gross amounts offset in the balance sheet	0	0
Total recognized assets, net amounts presented in the balance sheet	1,789	2,435
Total recognized assets, gross amounts not offset in the balance sheet, financial instruments	(106)	(109)
Total recognized assets, gross amounts not offset in the balance sheet, collateral received or pledged (including cash)	(1,661)	(2,295)
Total recognized assets, net amount	22	31
Offsetting Securities Loaned [Abstract]
Deposits received for securities loaned, gross	2,394	3,189
Offsetting Derivative Liability, Securities Sold under Agreements to Repurchase, Securities Loaned [Abstract]
Total recognized liabilities, gross	2,411	3,192
Total recognized liabilities, gross amounts offset in the balance sheet	0	0
Total recognized liabilities, net amounts presented in the balance sheet	2,411	3,192
Total recognized liabilities, gross amounts not offset in the balance sheet, financial instruments	(77)	(41)
Total recognized liabilities, gross amounts not offset in the balance sheet, collateral received or pledged (including cash)	(2,118)	(2,821)
Total recognized liabilities, net amount	216	330
Investments segregated for regulatory purposes [Member]
Offsetting Securities Purchased under Agreements to Resell [Abstract]
Reverse repurchase agreements, gross	500	500
Reverse repurchase agreements, gross amounts offset in the balance sheet	0	0
Reverse repurchase agreements, net amounts presented in the balance sheet	500	500
Reverse repurchase agreements, gross amounts not offset in the balance sheet, financial instruments	0	0
Reverse repurchase agreements, gross amounts not offset in the balance sheet, collateral received or pledged (including cash)	(500)	(500)
Reverse repurchase agreements, net amount	0	0
Receivable from brokers, dealers and clearing organizations [Member]
Offsetting Securities Borrowed [Abstract]
Deposits paid for securities borrowed, gross	1,244	1,864
Deposits paid for securities borrowed, gross amounts offset in the balance sheet	0	0
Deposits paid for securities borrowed, net amounts presented in the balance sheet	1,244	1,864
Deposits paid for securities borrowed, gross amounts not offset in the balance sheet, financial instruments	(61)	(38)
Deposits paid for securities borrowed, gross amounts not offset in the balance sheet, collateral received or pledged (including cash)	(1,161)	(1,795)
Deposits paid for securities borrowed, net amount	22	31
Other assets [Member]
Offsetting Derivative Assets [Abstract]
Pay-variable interest rate swaps, gross	45	71
Pay-variable interest rate swaps, gross amounts offset in the balance sheet	0	0
Pay-variable interest rate swaps, net amounts presented in the balance sheet	45	71
Pay-variable interest rate swaps, gross amounts not offset in the balance sheet, financial instruments	(45)	(71)
Pay-variable interest rate swaps, gross amounts not offset in the balance sheet, collateral received or pledged (including cash)	0	0
Pay-variable interest rate swaps, net amount	0	0
Payable to brokers, dealers and clearing organizations [Member]
Offsetting Securities Loaned [Abstract]
Deposits received for securities loaned, gross	2,394	3,189
Deposits received for securities loaned, gross amounts offset in the balance sheet	0	0
Deposits received for securities loaned, net amounts presented in the balance sheet	2,394	3,189
Deposits received for securities loaned, gross amounts not offset in the balance sheet, financial instruments	(61)	(38)
Deposits received for securities loaned, gross amounts not offset in the balance sheet, collateral received or pledged (including cash)	(2,118)	(2,821)
Deposits received for securities loaned, net amount	215	330
Accounts payable and other liabilities [Member]
Offsetting Derivative Liabilities [Abstract]
Pay-variable rate interest rate swaps, gross	17	3
Pay-variable rate interest rate swaps, gross amounts offset in the balance sheet	0	0
Pay-variable rate interest rate swaps, net amounts presented in the balance sheet	17	3
Pay-variable rate interest rate swaps, gross amounts not offset in the balance sheet, financial instruments	(16)	(3)
Pay-variable rate interest rate swaps, gross amounts not offset in the balance sheet, collateral received or pledged (including cash)	0	0
Pay-variable rate interest rate swaps, net amount	$ 1	$ 0