Revenues - Disaggregation of Revenue (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]
Revenues	$ 132,894	$ 140,489	$ 277,460	$ 271,207
Investment management fees
Disaggregation of Revenue [Line Items]
Revenues	110,550	114,591	230,838	220,509
Open-end funds
Disaggregation of Revenue [Line Items]
Revenues	54,018	56,973	113,126	110,266
Closed-end funds
Disaggregation of Revenue [Line Items]
Revenues	8,557	10,620	18,736	20,639
Retail separate accounts
Disaggregation of Revenue [Line Items]
Revenues	22,398	20,664	48,112	38,669
Institutional accounts
Disaggregation of Revenue [Line Items]
Revenues	24,606	23,656	47,523	45,833
Structured products
Disaggregation of Revenue [Line Items]
Revenues	417	1,585	1,991	3,232
Other products
Disaggregation of Revenue [Line Items]
Revenues	554	1,093	1,350	1,870
Distribution and service fees
Disaggregation of Revenue [Line Items]
Revenues	8,889	10,617	18,349	20,680
Administration and shareholder service fees
Disaggregation of Revenue [Line Items]
Revenues	13,289	15,054	27,942	29,467
Other income and fees
Disaggregation of Revenue [Line Items]
Revenues	$ 166	$ 227	$ 331	$ 551