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Acquisitions Gain Capital (Details) - USD ($)
$ / shares in Units, $ in Millions
3 Months Ended 9 Months Ended
Jun. 30, 2020
Jun. 30, 2020
Sep. 30, 2019
Jun. 30, 2019
Business Acquisition [Line Items]        
Debt Instrument, Face Amount $ 350.0 $ 350.0    
Debt Instrument, Interest Rate, Stated Percentage 8.625% 8.625%    
Restricted cash held in escrow $ 350.0 $ 350.0 $ 0.0 $ 0.0
Convertible Notes Payable $ 92.0 $ 92.0    
Gain Capital Holdings Inc [Member]        
Business Acquisition [Line Items]        
Business Acquisition, Share Price $ 6.00 $ 6.00    
Business Combination, Consideration Transferred   $ 236.2    
Restricted cash held in escrow $ 349.8 349.8    
Business Acquisition, Transaction Costs $ 1.4 $ 1.9