Investments - Narrative (Details) - USD ($) $ / shares in Units, $ in Millions		3 Months Ended		6 Months Ended
	Jul. 19, 2018	Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2020	Mar. 31, 2019	Sep. 30, 2019
Subsidiary, Sale of Stock [Line Items]
Equity securities		$ 647.7		$ 647.7		$ 721.5
Change in fair value of investments				(73.8)	$ (114.5)
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
Purchase of stock purchase period	3 years
Sale of stock, subsequent trading period	90 days
Equity securities		647.7		647.7
Change in fair value of investments		$ (144.8)	$ 98.2	$ (73.8)	$ (114.5)
PTC | Minimum
Subsidiary, Sale of Stock [Line Items]
Percentage of total outstanding common stock, that can be traded in future periods	1.00%
PTC | Maximum
Subsidiary, Sale of Stock [Line Items]
Percentage of total outstanding common stock, that can be traded in future periods	2.00%