Revenues - Disaggregation of Revenue (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Disaggregation of Revenue [Line Items]
Revenues	$ 145,955	$ 152,210	$ 417,162	$ 414,170
Investment management fees
Disaggregation of Revenue [Line Items]
Revenues	120,023	121,713	340,532	325,357
Open-end funds
Disaggregation of Revenue [Line Items]
Revenues	59,060	62,466	169,326	174,032
Closed-end funds
Disaggregation of Revenue [Line Items]
Revenues	10,846	10,614	31,485	31,161
Retail separate accounts
Disaggregation of Revenue [Line Items]
Revenues	22,092	19,532	60,761	53,152
Institutional accounts
Disaggregation of Revenue [Line Items]
Revenues	25,180	24,614	71,013	56,210
Structured products
Disaggregation of Revenue [Line Items]
Revenues	1,725	3,602	4,957	7,996
Other products
Disaggregation of Revenue [Line Items]
Revenues	1,120	885	2,990	2,806
Distribution and service fees
Disaggregation of Revenue [Line Items]
Revenues	10,442	13,730	31,122	39,886
Administration and shareholder service fees
Disaggregation of Revenue [Line Items]
Revenues	15,280	16,567	44,747	48,272
Other income and fees
Disaggregation of Revenue [Line Items]
Revenues	$ 210	$ 200	$ 761	$ 655