Revenues - Disaggregation of Revenue (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Disaggregation of Revenue [Line Items]
Revenues	$ 252,064	$ 154,790	$ 712,972	$ 432,250
Investment management fees
Disaggregation of Revenue [Line Items]
Revenues	201,133	129,785	567,912	360,623
Open-end funds
Disaggregation of Revenue [Line Items]
Revenues	99,278	63,458	284,945	176,584
Closed-end funds
Disaggregation of Revenue [Line Items]
Revenues	17,116	8,959	46,451	27,695
Retail separate accounts
Disaggregation of Revenue [Line Items]
Revenues	46,625	26,412	126,612	74,524
Institutional accounts
Disaggregation of Revenue [Line Items]
Revenues	36,728	29,048	104,948	76,571
Structured products
Disaggregation of Revenue [Line Items]
Revenues	953	1,297	3,797	3,288
Other products
Disaggregation of Revenue [Line Items]
Revenues	$ 433	$ 611	$ 1,159	$ 1,961