v3.20.4
Amounts Held for Sale (Details) - USD ($)
$ in Millions
3 Months Ended
Dec. 31, 2020
Sep. 30, 2020
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]    
(Gain) loss on assets held for sale $ (0.6)  
Level 3    
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]    
(Gain) loss on assets held for sale (0.6)  
Post Consumer Brands | Clinton, Massachusetts facility    
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]    
Property, net 1.4 $ 3.4
(Gain) loss on assets held for sale 0.1  
Post Consumer Brands | Asheboro, North Carolina facility    
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]    
Property, net   1.4
Weetabix Segment | Corby, United Kingdom facility    
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]    
Property, net   $ 2.5
(Gain) loss on assets held for sale $ (0.7)