Investments - Narrative (Details) - USD ($) $ / shares in Units, $ in Millions		12 Months Ended
	May 11, 2021	Sep. 30, 2021	Sep. 30, 2020	Jul. 19, 2018
Debt and Equity Securities, FV-NI [Line Items]
Equity securities		$ 1,267.6	$ 875.3
PTC
Debt and Equity Securities, FV-NI [Line Items]
Number of shares purchased (in shares)				10,582,010
Purchase of stock price (usd per share)				$ 94.50
Consideration paid to purchase stock				$ 1,000.0
Sale of stock, subsequent trading period	90 days
Sale of stock, trading period, marketed underwritten offerings	1 year
Gain on fair value of PTC shares		$ 392.3	$ 153.9
PTC | Minimum
Debt and Equity Securities, FV-NI [Line Items]
Percentage of total outstanding common stock, that can be traded in future periods	100.00%
Sale of stock, percentage of total outstanding common stock that may be transferred, marketed underwritten offerings	500.00%