v3.20.4

Subsequent Events (Details) - USD ($) $ in Millions | Feb. 01, 2021 | Jan. 25, 2021 | Dec. 31, 2020 | Dec. 31, 2019
--- | --- | --- | --- | ---
8th Avenue |  |  |  |
Subsequent Event [Line Items] |  |  |  |
Equity Method Investment, Ownership Percentage |  |  | 60.50% | 60.50%
Peter Pan | Subsequent Event |  |  |  |
Subsequent Event [Line Items] |  |  |  |
Payments to Acquire Businesses, Base Purchase Price |  | $ 102.0 |  |
Payment at closing |  | $ 103.4 |  |
Almark Foods | Subsequent Event |  |  |  |
Subsequent Event [Line Items] |  |  |  |
Payments to Acquire Businesses, Base Purchase Price | $ 52.0 |  |  |
Payment at closing | $ 51.3 |  |  |

X
- Definition Payments to Acquire Businesses, Base Purchase Price + References No definition available. + Details
Name: | post_PaymentsToAcquireBusinessesBasePurchasePrice
Namespace Prefix: | post_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition The percentage of ownership of common stock or equity participation in the investee accounted for under the equity method of accounting. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 323 -SubTopic 10 -Section 50 -Paragraph 3 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=114001798&loc=d3e33918-111571 + Details
Name: | us-gaap_EquityMethodInvestmentOwnershipPercentage
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition The cash outflow associated with the acquisition of business during the period. The cash portion only of the acquisition price. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 30 -Section 50 -Paragraph 1 -Subparagraph (b)(1) -URI http://asc.fasb.org/extlink&oid=120321790&loc=d3e6927-128479 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 13 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=121586364&loc=d3e3213-108585 + Details
Name: | us-gaap_PaymentsToAcquireBusinessesGross
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Detail information of subsequent event by type. User is expected to use existing line items from elsewhere in the taxonomy as the primary line items for this disclosure, which is further associated with dimension and member elements pertaining to a subsequent event. + References No definition available. + Details
Name: | us-gaap_SubsequentEventLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Details
Name: | srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis=post_A8thAvenueMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_BusinessAcquisitionAxis=post_PeterPanMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_SubsequentEventTypeAxis=us-gaap_SubsequentEventMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_BusinessAcquisitionAxis=post_AlmarkFoodsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |