Noncontrolling Interests, Equity Interests and Related Party Transactions (Notes) |
3 Months Ended |
|---|---|
Dec. 31, 2025 | |
| Equity Method Investments and Joint Ventures [Abstract] | |
| Equity Method Investments and Joint Ventures Disclosure - 8th Ave | 8th Avenue Prior to July 1, 2025, the Company had a 60.5% common equity interest in 8th Avenue Food & Provisions, Inc. (“8th Avenue”) that was accounted for using the equity method and transactions between the Company and 8th Avenue were considered related party transactions. On July 1, 2025, the Company completed its acquisition of all of the preferred stock and the remaining 39.5% common equity interest that it did not already own in 8th Avenue. Subsequent to the acquisition, the results of 8th Avenue were consolidated with the results of Post and transactions between Post and 8th Avenue were no longer classified as related party transactions. See Note 4 for additional information regarding the acquisition. There was no equity method earnings/loss attributable to 8th Avenue recognized during fiscal 2025 prior to the acquisition. During the three months ended December 31, 2024, the Company had net sales to 8th Avenue of $2.4 and purchases from and royalties paid to 8th Avenue of $20.3. Sales and purchases between the Company and 8th Avenue prior to the acquisition were all made at arm’s-length. |
| Equity Method Investments and Joint Ventures Disclosure - WBX | Weetabix East Africa and Alpen The Company holds a controlling equity interest in Weetabix East Africa Limited (“Weetabix East Africa”). Weetabix East Africa is a Kenyan-based company that produces ready-to-eat (“RTE”) cereal and muesli. The Company owns 50.2% of Weetabix East Africa and holds a controlling voting and financial interest through its appointment of management and representation on Weetabix East Africa’s board of directors. Accordingly, Weetabix East Africa is fully consolidated into the Company’s financial statements and its assets and results of operations are reported in the Weetabix segment. The remaining interest in the consolidated net earnings and net assets of Weetabix East Africa is allocated to noncontrolling interest. The Company holds an equity interest in Alpen Food Company South Africa (Pty) Limited (“Alpen”). Alpen is a South African-based company that produces RTE cereal and muesli. The Company owns 50.0% of Alpen’s common stock with no other indicators of control, and accordingly, the Company accounts for its investment in Alpen using the equity method. The investment in Alpen was $5.1 and $4.5 at December 31, 2025 and September 30, 2025, respectively, and was included in “Other assets” on the Condensed Consolidated Balance Sheets. |
| Related Party Transactions Disclosure-BRBR | BellRing Transactions between the Company and BellRing Brands, Inc. (“BellRing”) are considered related party transactions as certain of the Company’s officers and/or directors serve as officers and/or directors of BellRing. The Company has a co-packing agreement with BellRing under which Post produces protein-based shakes for BellRing and BellRing procures certain packaging materials for Post to utilize in its production. Net sales to BellRing were $18.1 during the three months ended December 31, 2025 and were immaterial during the three months ended December 31, 2024. The Company had current receivables with BellRing of $6.6 and $3.4 related to these sales at December 31, 2025 and September 30, 2025, respectively, which were included in “Receivables, net” on the Condensed Consolidated Balance Sheets. Other related party transactions between the Company and BellRing were immaterial for the three months ended December 31, 2025 and 2024, and balances related to such transactions were immaterial as of both December 31, 2025 and September 30, 2025. |