Revenue from Contracts with Customers - Disaggregation of Revenue by Asset Class (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Disaggregation of Revenue [Line Items]
Total Revenue	$ 307,216	$ 308,616	$ 255,993	$ 263,852	$ 278,312	$ 278,315	$ 272,796	$ 273,501	$ 1,135,677	$ 1,102,924	$ 1,143,371
Equity
Disaggregation of Revenue [Line Items]
Total Revenue									470,436
Money Market
Disaggregation of Revenue [Line Items]
Total Revenue									414,746
Fixed-Income
Disaggregation of Revenue [Line Items]
Total Revenue									180,152
Other
Disaggregation of Revenue [Line Items]
Total Revenue									$ 70,343