Investments - Gains and Losses on Investments within Change in Fair Value (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2023
Investments, Debt and Equity Securities [Abstract]
Net gain on equity securities (level 1)	$ 0.0	$ 0.0	$ 281.0
Net loss on equity securities (other)	(3.0)	(1.0)	(1.0)
Equity method gain (loss) on Other investments	0.0	1.0	(1.0)
Change in fair value of investments	(3.0)	0.0	279.0
Total net realized gain on equity securities	0.0	0.0	281.0
Total net unrealized loss on equity securities	$ (3.0)	$ (1.0)	$ (1.0)