| Document and Entity Information | Feb. 28, 2022 |
| Cover [Abstract] | |
| Entity Address, State or Province | MO |
| Amendment Flag | false |
| Entity Central Index Key | 0001530950 |
| Document Type | 8-K |
| Document Period End Date | Feb. 28, 2022 |
| Entity Registrant Name | Post Holdings, Inc. |
| Entity Incorporation State 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, Postal Zip Code | 63144 |
| City Area Code | (314) |
| Local Phone Number | 644-7600 |
| Written Communications | true |
| Soliciting Material | false |
| Pre Commencement Tender Offer | false |
| Pre Commencement Issuer Tender Offer | false |
| Security 12b Title | Common Stock, $0.01 par value per share |
| Trading Symbol | POST |
| Security Exchange Name | NYSE |
| Entity Emerging Growth Company | false |
