Investments - Gains and Losses on Investments within Change in Fair Value (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Investments, Debt and Equity Securities [Abstract]
Net gain (loss) on equity securities (level 1)	$ 86.4	$ (8.4)	$ 290.4	$ (136.0)
Net gain on equity securities (other)	0.0	3.2	0.0	11.9
Equity method loss on Other investments	(0.7)	0.0	(1.1)	(14.2)
Change in fair value of investments	85.7	(5.2)	289.3	(138.3)
Total net realized gain on equity securities	36.9	12.7	89.8	12.9
Total net unrealized gain (loss) on equity securities	$ 49.5	$ (17.9)	$ 200.6	$ (137.0)