Revenue (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Disaggregation of Revenue [Line Items]
Total revenue	$ 410,830	$ 381,111	$ 378,696
North America
Disaggregation of Revenue [Line Items]
Total revenue	352,629	322,964	314,021
Japan
Disaggregation of Revenue [Line Items]
Total revenue	33,967	35,283	42,303
Asia Pacific, excluding Japan
Disaggregation of Revenue [Line Items]
Total revenue	13,147	12,493	11,496
Europe, Middle East and Africa
Disaggregation of Revenue [Line Items]
Total revenue	11,087	10,371	10,876
Distribution and service fees
Disaggregation of Revenue [Line Items]
Total revenue	30,048	29,090	30,747
Open-end funds
Disaggregation of Revenue [Line Items]
Total revenue	217,778	197,363	193,348
Closed-end funds
Disaggregation of Revenue [Line Items]
Total revenue	80,502	77,270	78,670
Institutional accounts
Disaggregation of Revenue [Line Items]
Total revenue	110,346	104,327	103,629
Other (2)
Disaggregation of Revenue [Line Items]
Total revenue	$ 2,204	$ 2,151	$ 3,049