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Revenue from Contracts with Customers - Disaggregation of Revenue by Asset Class (Details) - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2019
Mar. 31, 2018
Disaggregation of Revenue [Line Items]    
Revenue $ 307,050 $ 263,852
Equity    
Disaggregation of Revenue [Line Items]    
Revenue 123,634 103,609
Money Market    
Disaggregation of Revenue [Line Items]    
Revenue 117,307 104,483
Fixed-Income    
Disaggregation of Revenue [Line Items]    
Revenue 43,677 45,048
Other    
Disaggregation of Revenue [Line Items]    
Revenue $ 22,432 $ 10,712