Revenues - Disaggregation of Revenue (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Disaggregation of Revenue [Line Items]
Revenues	$ 244,011	$ 132,894	$ 460,908	$ 277,460
Investment management fees
Disaggregation of Revenue [Line Items]
Revenues	193,510	110,550	366,779	230,838
Open-end funds
Disaggregation of Revenue [Line Items]
Revenues	96,795	54,018	185,667	113,126
Closed-end funds
Disaggregation of Revenue [Line Items]
Revenues	16,395	8,557	29,335	18,736
Retail separate accounts
Disaggregation of Revenue [Line Items]
Revenues	42,475	22,398	79,987	48,112
Institutional accounts
Disaggregation of Revenue [Line Items]
Revenues	35,782	24,606	68,220	47,523
Structured products
Disaggregation of Revenue [Line Items]
Revenues	1,585	417	2,844	1,991
Other products
Disaggregation of Revenue [Line Items]
Revenues	$ 478	$ 554	$ 726	$ 1,350