Revenue from Contracts with Customers - Disaggregation of Revenue by Asset Class (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Disaggregation of Revenue [Line Items]
Total Revenue	$ 360,712	$ 321,479	$ 719,895	$ 628,529
Money Market
Disaggregation of Revenue [Line Items]
Total Revenue	157,430	123,136	308,850	240,443
Equity
Disaggregation of Revenue [Line Items]
Total Revenue	123,328	134,035	260,618	257,669
Fixed-Income
Disaggregation of Revenue [Line Items]
Total Revenue	43,691	44,782	90,297	88,459
Other
Disaggregation of Revenue [Line Items]
Total Revenue	$ 36,263	$ 19,526	$ 60,130	$ 41,958