Related Party Transactions - Summary of Classification and Amount of Receivables from and Payables to Affiliates of Company (Details) - USD ($) $ in Millions	Dec. 31, 2019	Sep. 30, 2019
Assets:
Receivable from brokers, dealers and clearing organizations	$ 1,728	$ 2,439
Receivable from affiliates	121	112
Liabilities:
Payable to brokers, dealers and clearing organizations	2,482	3,308
Payable to affiliates	3	5
TD and its affiliates [Member]
Assets:
Receivable from brokers, dealers and clearing organizations	372	0
Receivable from affiliates	121	112
Liabilities:
Payable to brokers, dealers and clearing organizations	37	44
Payable to affiliates	3	5
Accounts payable and other liabilities	$ 2	$ 2