Investments - Narrative (Details) - USD ($) $ / shares in Units, $ in Millions		3 Months Ended		9 Months Ended
	May 11, 2021	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020	Sep. 30, 2020	Jul. 19, 2018
Subsidiary, Sale of Stock [Line Items]
Equity securities		$ 1,494.8		$ 1,494.8		$ 875.3
Change in fair value of investments				624.6	$ 101.7
PTC
Subsidiary, Sale of Stock [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, subsequent trading period, marketed underwritten offerings	1 year
Equity securities		1,494.8		1,494.8
Change in fair value of investments		$ 38.2	$ 175.5	$ 619.5	$ 101.7
PTC | Minimum
Subsidiary, Sale of Stock [Line Items]
Percentage of total outstanding common stock, that can be traded in future periods	1.00%
Percentage of total outstanding common stock that can be traded in future periods, marketed underwritten offerings	5.00%