Investments - Gains and Losses on Investments within Change in Fair Value (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Investments, Debt and Equity Securities [Abstract]
Net gain on equity securities (level 1)	$ 0.0	$ 86.4	$ 0.0	$ 290.4
Net loss on equity securities (other)	(5.0)	0.0	0.0	0.0
Equity method (loss) gain on Other investments	0.0	(0.7)	0.9	(1.1)
Change in fair value of investments	(5.0)	85.7	0.9	289.3
Total net realized gain on equity securities	0.0	36.9	0.0	89.8
Total net unrealized (loss) gain on equity securities	$ (5.0)	$ 49.5	$ 0.0	$ 200.6