Investments - Gains and Losses on Investments within Change in Fair Value (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2024	Mar. 31, 2023	Mar. 31, 2024	Mar. 31, 2023
Investments, Debt and Equity Securities [Abstract]
Net gain on equity securities (level 1)	$ 0.0	$ 63.0	$ 0.0	$ 204.0
Net gain on equity securities (other)	2.5	0.0	5.0	0.0
Equity method gain (loss) on Other investments	0.3	0.0	0.9	(0.4)
Change in fair value of investments	2.8	63.0	5.9	203.6
Total net realized gain on equity securities	0.0	19.0	0.0	52.9
Total net unrealized gain on equity securities	$ 2.5	$ 44.0	$ 5.0	$ 151.1