Investments - Gains and Losses on Investments within Change in Fair Value (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2021
Investments, Debt and Equity Securities [Abstract]
Net gain (loss) on equity securities (level 1)	$ 281.7	$ (136.4)	$ 392.3
Net (loss) gain on equity securities (other)	(1.3)	15.1	5.1
Equity method loss on Other investments	(1.1)	(15.6)	0.0
Change in fair value of investments	279.3	(136.9)	397.4
Total net realized gain on equity securities	281.7	44.6	0.0
Total net unrealized (loss) gain on equity securities	$ (1.3)	$ (165.9)	$ 397.4