Related Party Transactions (Policies)	12 Months Ended
Sep. 30, 2021
Related Party Transactions [Abstract]
Related Party Transactions, Policy	Prior to the IPO, the Company used certain functions and services performed by Post. These functions and services included legal, finance, internal audit, treasury, information technology support, insurance and tax matters, the use of office and/or data center space, payroll processing services and tax compliance services. Costs for these functions and services performed by Post were allocated to the Company based on a reasonable activity base (including specific costs, revenue, net assets and headcount, or a combination of such items) or another reasonable method. For the year ended September 30, 2019, allocated costs were $12.6, which included $6.7 of costs related to the separation from Post. Costs related to the separation from Post were $0.2 and $1.9 for the years ended September 30, 2021 and 2020, respectively. Allocated costs and costs related to the separation from Post were included in “Selling, general and administrative expenses” in the Consolidated Statements of Operations. After the completion of the IPO, Post continues to provide these services and other services to the Company under a master services agreement (the “MSA”). In addition to charges for these services, the Company also incurs certain pass-through charges from Post, primarily relating to stock-based compensation for employees participating in Post’s stock-based compensation plans.

Related Party Transactions (Policies)

12 Months Ended

Sep. 30, 2021

Prior to the IPO, the Company used certain functions and services performed by Post. These functions and services included legal, finance, internal audit, treasury, information technology support, insurance and tax matters, the use of office and/or data center space, payroll processing services and tax compliance services. Costs for these functions and services performed by Post were allocated to the Company based on a reasonable activity base (including specific costs, revenue, net assets and headcount, or a combination of such items) or another reasonable method. For the year ended September 30, 2019, allocated costs were $12.6, which included $6.7 of costs related to the separation from Post. Costs related to the separation from Post were $0.2 and $1.9 for the years ended September 30, 2021 and 2020, respectively. Allocated costs and costs related to the separation from Post were included in “Selling, general and administrative expenses” in the Consolidated Statements of Operations. After the completion of the IPO, Post continues to provide these services and other services to the Company under a master services agreement (the “MSA”). In addition to charges for these services, the Company also incurs certain pass-through charges from Post, primarily relating to stock-based compensation for employees participating in Post’s stock-based compensation plans.