Investments - Gains and Losses on Investments within Change in Fair Value (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Mar. 31, 2023	Mar. 31, 2022	Mar. 31, 2023	Mar. 31, 2022
Investments, Debt and Equity Securities [Abstract]
Net gain (loss) on equity securities (level 1)	$ 63.0	$ (142.0)	$ 204.0	$ (127.6)
Net gain on equity securities (other)	0.0	8.7	0.0	8.7
Equity method loss on Other investments	0.0	(7.4)	(0.4)	(14.2)
Change in fair value of investments	63.0	(140.7)	203.6	(133.1)
Total net realized gain on equity securities	19.0	0.2	52.9	0.2
Total net unrealized gain (loss) on equity securities	$ 44.0	$ (133.5)	$ 151.1	$ (119.1)

Equity Securities Without Readily Determinable Fair Value, Gain (Loss)

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Amount of realized gain from sale of investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

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Amount of realized gain (loss) from sale of investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

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Amount of unrealized gain (loss) on investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

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Amount of realized and unrealized gain (loss) on investment.

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Amount of income (loss) for proportionate share of equity method investee's income (loss).

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