Supplemental Information (Shareholder Activism-Related Costs) (Details) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Jun. 30, 2026 USD ($)	Jun. 30, 2025 USD ($) director	Jun. 30, 2026 USD ($)	Jun. 30, 2025 USD ($)	Sep. 30, 2025 USD ($)
Restructuring Cost and Reserve [Line Items]
Legal, executive separation, and other professional service fees in connection with response to proxy contest	$ 0.0	$ 25.0	$ 0.0	$ 86.3
Shareholder activism-related costs, after tax		$ 18.8		$ 71.7
Professional fees					$ 31.9
CEO separation costs					29.7
Number of directors excluded from vote | director		1
Executive Separation Costs
Restructuring Cost and Reserve [Line Items]
Share-based payment arrangement, accelerated cost					22.4
Charge for severance and other postemployment benefits					$ 7.3
Mantle Ridge LP and Affiliates Transaction
Restructuring Cost and Reserve [Line Items]
Reimbursement for costs incurred in connection with the proxy contest		$ 24.7