Cover - USD ($)	12 Months Ended
	Sep. 30, 2020	Nov. 16, 2020	Mar. 31, 2020
Cover [Abstract]
Document Type	10-K
Document Annual Report	true
Document Period End Date	Sep. 30, 2020
Document Transition Report	false
Entity File Number	1-35305
Entity Registrant Name	POST HOLDINGS, INC.
Entity Incorporation, State or Country Code	MO
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
Title of 12(b) Security	Common Stock, $0.01 par value
Trading Symbol	POST
Security Exchange Name	NYSE
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
ICFR Auditor Attestation Flag	true
Entity Shell Company	false
Entity Public Float			$ 4,680,868,433
Entity Common Stock, Shares Outstanding (in shares)		65,848,095
Documents Incorporated by Reference	Certain portions of the registrant’s definitive proxy statement for its 2021 annual meeting of shareholders, to be filed with the Securities and Exchange Commission within 120 days after September 30, 2020, are incorporated by reference into Part III of this report.
Entity Central Index Key	0001530950
Current Fiscal Year End Date	--09-30
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	FY
Amendment Flag	false