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Acquisitions - Gain Capital Holdings Inc (Details) - USD ($)
$ / shares in Units, $ in Millions
12 Months Ended
Feb. 26, 2020
Sep. 30, 2020
Sep. 30, 2019
Aggregated Merger Consideration      
Common stock outstanding (in shares)   19,376,594 19,075,360
Gain Capital Holdings Inc [Member]      
Aggregated Merger Consideration      
Common stock outstanding (in shares) 3,600,000    
Gain Capital Holdings Inc [Member]      
Business Acquisition [Line Items]      
Business acquisition, share price (in dollars per share) $ 6.00    
Aggregated Merger Consideration      
Aggregate cash consideration $ 215.0    
Accrual for merger cash consideration 21.6    
Total merger consideration $ 236.6    
Business acquisition, cost incurred related to merger   $ 5.2