Revenues - Disaggregation of Revenue (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Disaggregation of Revenue [Line Items]
Revenues	$ 154,790	$ 145,955	$ 432,250	$ 417,162
Investment management fees
Disaggregation of Revenue [Line Items]
Revenues	129,785	120,023	360,623	340,532
Open-end funds
Disaggregation of Revenue [Line Items]
Revenues	63,458	59,060	176,584	169,326
Closed-end funds
Disaggregation of Revenue [Line Items]
Revenues	8,959	10,846	27,695	31,485
Retail separate accounts
Disaggregation of Revenue [Line Items]
Revenues	26,412	22,092	74,524	60,761
Institutional accounts
Disaggregation of Revenue [Line Items]
Revenues	29,048	25,180	76,571	71,013
Structured products
Disaggregation of Revenue [Line Items]
Revenues	1,297	1,725	3,288	4,957
Other products
Disaggregation of Revenue [Line Items]
Revenues	611	1,120	1,961	2,990
Distribution and service fees
Disaggregation of Revenue [Line Items]
Revenues	9,797	10,442	28,146	31,122
Administration and shareholder service fees
Disaggregation of Revenue [Line Items]
Revenues	15,114	15,280	43,056	44,747
Other income and fees
Disaggregation of Revenue [Line Items]
Revenues	$ 94	$ 210	$ 425	$ 761