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Summary of Significant Accounting Policies (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Property and Equipment and Capitalized Software      
Capitalized software development costs, excluding charges recognized in relation to the IPO $ 15,700 $ 11,100 $ 10,100
Amortization expense for capitalized software, excluding charges recognized in relation to the IPO $ 10,100 10,100 9,600
Securities Borrowed and Securities Loaned      
Minimum initial collateral advanced or received expressed as a percentage of fair value of the underlying securities borrowed or loaned 102.00%    
Class B interests      
Property and Equipment and Capitalized Software      
Capitalized software development costs recognized upon the IPO $ 40 90 9,200
Class B interests | East MIP      
Property and Equipment and Capitalized Software      
Amortization expense for capitalized software recognized upon the IPO $ 70 $ 700 $ 8,500
Furniture, fixtures and equipment | Minimum      
Property and Equipment and Capitalized Software      
Estimated useful lives 3 years    
Furniture, fixtures and equipment | Maximum      
Property and Equipment and Capitalized Software      
Estimated useful lives 7 years    
Capitalized software | Minimum      
Property and Equipment and Capitalized Software      
Estimated useful lives 1 year 6 months    
Capitalized software | Maximum      
Property and Equipment and Capitalized Software      
Estimated useful lives 2 years 6 months