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Investments - Narrative (Details) - USD ($)
$ / shares in Units, $ in Millions
3 Months Ended 9 Months Ended
Jul. 19, 2018
Jun. 30, 2020
Jun. 30, 2019
Jun. 30, 2020
Jun. 30, 2019
Sep. 30, 2019
Subsidiary, Sale of Stock [Line Items]            
Equity securities   $ 823.2   $ 823.2   $ 721.5
Change in fair value of investments       101.7 $ (140.1)  
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   823.2   823.2    
Change in fair value of investments   $ 175.5 $ (25.6) $ 101.7 $ (140.1)  
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%