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Acquisitions - Gain Capital Holdings Inc (Details) - USD ($)
$ / shares in Units, $ in Millions
1 Months Ended 12 Months Ended
Feb. 29, 2020
Sep. 30, 2021
Sep. 30, 2020
Aggregated Merger Consideration      
Common stock outstanding (in shares)   19,823,910 19,376,594
Gain Capital Holdings Inc      
Aggregated Merger Consideration      
Common stock outstanding (in shares) 3,600,000    
Gain Capital Holdings Inc      
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 $ 236.6  
Business acquisition, cost incurred related to merger     $ 5.2