Investments - Gains and Losses on Investments within Change in Fair Value (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2022
Investments, Debt and Equity Securities [Abstract]
Net gain (loss) on equity securities (level 1)	$ 0.0	$ 281.7	$ (136.4)
Net (loss) gain on equity securities (other)	(0.8)	(1.3)	15.1
Equity method gain (loss) on Other investments	0.9	(1.1)	(15.6)
Change in fair value of investments	0.1	279.3	(136.9)
Total net realized gain on equity securities	0.0	281.7	44.6
Total net unrealized loss on equity securities	$ (0.8)	$ (1.3)	$ (165.9)