Revenue from Contracts with Customers - Disaggregation of Revenue by Asset Class (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Disaggregation of Revenue [Line Items]
Total Revenue	$ 364,455	$ 340,340	$ 1,084,350	$ 968,869
Money Market
Disaggregation of Revenue [Line Items]
Total Revenue	143,391	138,242	452,240	378,686
Equity
Disaggregation of Revenue [Line Items]
Total Revenue	138,590	136,655	399,208	394,324
Fixed-Income
Disaggregation of Revenue [Line Items]
Total Revenue	49,407	45,018	139,705	133,476
Other
Disaggregation of Revenue [Line Items]
Total Revenue	$ 33,067	$ 20,425	$ 93,197	$ 62,383