Supplemental Information (Shareholder Activism-Related Costs) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2026	Mar. 31, 2025	Mar. 31, 2026	Mar. 31, 2025
Restructuring Cost and Reserve [Line Items]
Legal, executive separation, and other professional service fees in connection with response to proxy contest	$ 0.0	$ 31.4	$ 0.0	$ 61.3
Shareholder activism-related costs, after tax		31.0		52.9
Charge for severance and other postemployment benefits				66.1
Executive Separation Costs
Restructuring Cost and Reserve [Line Items]
Share-based payment arrangement, accelerated cost		22.4		$ 22.4
Charge for severance and other postemployment benefits		$ 7.3