v3.20.4
Cover Page - shares
 3 Months Ended
Dec. 31, 2020
Feb. 01, 2021
Cover [Abstract]    
Document Type 10-Q  
Document Quarterly Report true  
Document Period End Date Dec. 31, 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 Current Reporting Status Yes  
Entity Interactive Data Current Yes  
Entity Filer Category Large Accelerated Filer  
Entity Small Business false  
Entity Emerging Growth Company false  
Entity Shell Company false  
Entity Common Stock, Shares Outstanding (in shares)   64,367,518
Entity Central Index Key 0001530950  
Current Fiscal Year End Date --09-30  
Document Fiscal Year Focus 2021  
Document Fiscal Period Focus Q1  
Amendment Flag false