Long-term Debt and Other Borrowings - Senior Notes, Lines of Credit and Securities Sold Under Agreements to Repurchase - Additional Information (Details)	Dec. 31, 2019 USD ($) bank	Sep. 30, 2019 USD ($)
Debt Instrument [Line Items]
Face Value	$ 3,550,000,000	$ 3,550,000,000
Borrowings outstanding	$ 3,555,000,000	3,594,000,000
Line of Credit [Member] | TD Ameritrade Clearing, Inc. [Member]
Debt Instrument [Line Items]
Number of unaffiliated banks | bank	2
Borrowings outstanding	$ 0	0
Senior Notes [Member]
Debt Instrument [Line Items]
Face Value	$ 3,550,000,000	$ 3,550,000,000