Related Party Transactions (Policies)	12 Months Ended
Sep. 30, 2020
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 years ended September 30, 2019 and 2018, allocated costs were $12.6 and $4.6, respectively, including $6.7 and zero, respectively, of costs related to the separation from Post. Allocated costs were included in “Selling, general and administrative expenses” in the Consolidated Statements of Operations. Costs related to the separation from Post were $1.9 for the year ended September 30, 2020.After the completion of the IPO, Post continues to provide these services and other services to the Company under a master services agreement (“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, 2020

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 years ended September 30, 2019 and 2018, allocated costs were $12.6 and $4.6, respectively, including $6.7 and zero, respectively, of costs related to the separation from Post. Allocated costs were included in “Selling, general and administrative expenses” in the Consolidated Statements of Operations. Costs related to the separation from Post were $1.9 for the year ended September 30, 2020.After the completion of the IPO, Post continues to provide these services and other services to the Company under a master services agreement (“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.