Related Party Transactions - Summary of Expenses Resulting from Transactions with Related Parties (Details) - TD and its affiliates [Member] - USD ($) $ in Millions	3 Months Ended
Dec. 31, 2019	Dec. 31, 2018
Related Party Transaction [Line Items]
Expenses to related parties	$ 6	$ 7
Order Routing Agreement [Member] \| Other Expense [Member]
Related Party Transaction [Line Items]
Expenses to related parties	4	5
Other [Member] \| Various [Member]
Related Party Transaction [Line Items]
Expenses to related parties	$ 2	$ 2

Expenses recognized resulting from transactions (excluding transactions that are eliminated in consolidated or combined financial statements) with related party.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 850
-SubTopic 10
-Section 50
-Paragraph 1
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=6457730&loc=d3e39549-107864

Name:	us-gaap_RelatedPartyTransactionExpensesFromTransactionsWithRelatedParty
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration