| Cover | Sep. 30, 2020 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Amendment Flag | false |
| Document Period End Date | Sep. 30, 2020 |
| Entity File Number | 1-35509 |
| Entity Registrant Name | TD Ameritrade Holding Corporation |
| Entity Central Index Key | 0001173431 |
| Entity Tax Identification Number | 82-0543156 |
| Entity Incorporation, State or Country Code | DE |
| Entity Address, Address Line One | 200 South 108th Avenue |
| Entity Address, City or Town | Omaha |
| Entity Address, State or Province | NE |
| Entity Address, Postal Zip Code | 68154 |
| City Area Code | 800 |
| Local Phone Number | 669-3900 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Title of 12(b) Security | Common Stock - $0.01 par value |
| Trading Symbol | AMTD |
| Security Exchange Name | NASDAQ |
| Entity Emerging Growth Company | false |
