Revenue from Contracts with Customers - Disaggregation of Revenue by Asset Class (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Disaggregation of Revenue [Line Items]
Total Revenue	$ 359,183	$ 307,050
Equity
Disaggregation of Revenue [Line Items]
Total Revenue	137,290	123,634
Money Market
Disaggregation of Revenue [Line Items]
Total Revenue	151,420	117,307
Fixed-Income
Disaggregation of Revenue [Line Items]
Total Revenue	46,607	43,677
Other
Disaggregation of Revenue [Line Items]
Total Revenue	$ 23,866	$ 22,432