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Commitments and Contingent Liabilities - Additional Information (Details)
€ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2016
USD ($)
compensation_guarantee
Sep. 30, 2015
USD ($)
Sep. 30, 2016
USD ($)
compensation_guarantee
loan
Sep. 30, 2015
USD ($)
Jul. 08, 2016
EUR (€)
Dec. 31, 2015
USD ($)
Loss Contingencies [Line Items]            
Compensation guarantees payable, number | compensation_guarantee 1   1      
Compensation guarantees payable           $ 0
Occupancy and Equipment Rentals            
Loss Contingencies [Line Items]            
Rental expense under the office leases $ 6,400,000 $ 4,000,000 $ 18,900,000 $ 12,900,000    
Revolving Credit Facility            
Loss Contingencies [Line Items]            
Weighted average interest rate 4.40%   4.40%      
Debt term     3 years      
Trading Activities on Euronext            
Loss Contingencies [Line Items]            
Loss contingency reserve | €         € 0.4  
Guarantee Obligation            
Loss Contingencies [Line Items]            
Letters of credit obligation $ 11,100,000   $ 11,100,000      
Indemnification Agreement            
Loss Contingencies [Line Items]            
Number of loans with contingent potential obligation | loan     40      
Indemnification Agreement | Maximum            
Loss Contingencies [Line Items]            
Potential losses on loans $ 8,500,000.0   $ 8,500,000.0