| Cover Page Document | Feb. 24, 2021 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Document Period End Date | Feb. 24, 2021 |
| Entity Registrant Name | Post Holdings, Inc. |
| Entity Incorporation, State or Country Code | MO |
| Entity File Number | 001-35305 |
| Entity Tax Identification Number | 45-3355106 |
| Entity Address, Address Line One | 2503 S. Hanley Road |
| Entity Address, City or Town | St. Louis |
| Entity Address, State or Province | MO |
| Entity Address, Postal Zip Code | 63144 |
| City Area Code | 314 |
| Local Phone Number | 644-7600 |
| 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 | POST |
| Security Exchange Name | NYSE |
| Entity Emerging Growth Company | false |
| Entity Central Index Key | 0001530950 |
| Document Fiscal Year Focus | |
| Document Fiscal Period Focus | |
| Amendment Flag | false |
