Revenues - Disaggregation of Revenue (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Disaggregation of Revenue [Line Items]
Revenues	$ 130,718	$ 129,028
Investment management fees
Disaggregation of Revenue [Line Items]
Revenues	105,918	100,476
Open-end funds
Disaggregation of Revenue [Line Items]
Revenues	53,293	54,361
Closed-end funds
Disaggregation of Revenue [Line Items]
Revenues	10,019	10,378
Retail separate accounts
Disaggregation of Revenue [Line Items]
Revenues	18,005	16,529
Institutional accounts
Disaggregation of Revenue [Line Items]
Revenues	22,177	15,818
Structured products
Disaggregation of Revenue [Line Items]
Revenues	1,647	2,326
Other products
Disaggregation of Revenue [Line Items]
Revenues	777	1,064
Distribution and service fees
Disaggregation of Revenue [Line Items]
Revenues	10,063	12,607
Administration and shareholder service fees
Disaggregation of Revenue [Line Items]
Revenues	14,413	15,738
Other income and fees
Disaggregation of Revenue [Line Items]
Revenues	$ 324	$ 207