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Revenue from Contracts with Customers (Details) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Revenue From External Customers by Asset Class [Line Items]        
Revenue from Contract with Customer, Excluding Assessed Tax $ 255,993 $ 272,796 $ 519,845 $ 546,297
Equity [Member]        
Revenue From External Customers by Asset Class [Line Items]        
Revenue from Contract with Customer, Excluding Assessed Tax 112,558   226,913  
Money Market [Member]        
Revenue From External Customers by Asset Class [Line Items]        
Revenue from Contract with Customer, Excluding Assessed Tax 98,923   203,406  
Fixed-Income [Member]        
Revenue From External Customers by Asset Class [Line Items]        
Revenue from Contract with Customer, Excluding Assessed Tax $ 44,512   $ 89,526