Revenues - Disaggregation of Revenue (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Disaggregation of Revenue [Line Items]
Revenues	$ 144,566	$ 130,718
Investment management fees
Disaggregation of Revenue [Line Items]
Revenues	120,288	105,918
Open-end funds
Disaggregation of Revenue [Line Items]
Revenues	59,108	53,293
Closed-end funds
Disaggregation of Revenue [Line Items]
Revenues	10,179	10,019
Retail separate accounts
Disaggregation of Revenue [Line Items]
Revenues	25,714	18,005
Institutional accounts
Disaggregation of Revenue [Line Items]
Revenues	22,917	22,177
Structured products
Disaggregation of Revenue [Line Items]
Revenues	1,574	1,647
Other products
Disaggregation of Revenue [Line Items]
Revenues	796	777
Distribution and service fees
Disaggregation of Revenue [Line Items]
Revenues	9,460	10,063
Administration and shareholder service fees
Disaggregation of Revenue [Line Items]
Revenues	14,653	14,413
Other income and fees
Disaggregation of Revenue [Line Items]
Revenues	$ 165	$ 324