Revenue - Schedule of Disaggregation of Revenue by Type and Product (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
Total revenue	$ 181,886	$ 91,360	$ 386,307	$ 178,839
Investment Management Fees
Disaggregation of revenue
Total revenue	130,032	78,042	276,913	152,453
Investment Management Fees | Mutual Funds
Disaggregation of revenue
Total revenue	104,858	52,351	224,372	102,927
Investment Management Fees | ETF's
Disaggregation of revenue
Total revenue	2,446	2,040	5,608	4,056
Investment Management Fees | Separate Accounts and Other Vehicles
Disaggregation of revenue
Total revenue	23,057	23,817	47,602	45,867
Performance-based Investment Fees | Separate Accounts and Other Vehicles
Disaggregation of revenue
Total revenue	(329)	(166)	(669)	(397)
Administration Fees | Mutual Funds
Disaggregation of revenue
Total revenue	26,733	5,391	56,343	10,636
Administration Fees | ETF's
Disaggregation of revenue
Total revenue	300	276	685	546
Fund Distribution Fees | Mutual Funds
Disaggregation of revenue
Total revenue	5,907	7,651	12,703	15,204
Transfer Agent Fees | Mutual Funds
Disaggregation of revenue
Total revenue	18,914		39,663
Fund Administration and Distribution Fees
Disaggregation of revenue
Total revenue	$ 51,854	$ 13,318	$ 109,394	$ 26,386