Related Party Transactions - Summary of Revenues 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]
Revenues from related parties	$ 433	$ 423
Insured Deposit Account Agreement [Member] \| Bank Deposit Account Fees [Member]
Related Party Transaction [Line Items]
Revenues from related parties	425	402
Order Routing Agreement [Member] \| Other Revenues [Member]
Related Party Transaction [Line Items]
Revenues from related parties	6	6
Other [Member] \| Various [Member]
Related Party Transaction [Line Items]
Revenues from related parties	$ 2	$ 15

Amount of revenue, fees and commissions earned from transactions between (a) a parent company and its subsidiaries; (b) subsidiaries of a common parent; (c) an entity and trusts for the benefit of employees, for example, but not limited to, pension and profit-sharing trusts that are managed by or under the trusteeship of the entity's management; (d) an entity and its principal, owners, management, or members of their immediate families; and (e) affiliates.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 946
-SubTopic 220
-Section S99
-Paragraph 1
-Subparagraph (SX 210.6-07.1(c))
-URI http://asc.fasb.org/extlink&oid=116637345&loc=SL114874292-224272

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.1(e))
-URI http://asc.fasb.org/extlink&oid=116634182&loc=SL114868664-224227

Name:	us-gaap_RevenueFromRelatedParties
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration