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]
Total revenue		$ 111,159	$ 104,965	$ 311,076	$ 300,983
Distribution and service fees
Disaggregation of Revenue [Line Items]
Total revenue		7,572	7,681	22,285	22,072
Open-end funds
Disaggregation of Revenue [Line Items]
Total revenue	[1]	59,176	56,220	168,333	158,680
Closed-end funds
Disaggregation of Revenue [Line Items]
Total revenue		18,676	20,714	55,810	59,768
Institutional accounts
Disaggregation of Revenue [Line Items]
Total revenue		32,880	27,510	85,239	81,045
Other
Disaggregation of Revenue [Line Items]
Total revenue	[2]	427	521	1,694	1,490
North America
Disaggregation of Revenue [Line Items]
Total revenue		91,298	90,674	263,712	258,384
Japan
Disaggregation of Revenue [Line Items]
Total revenue		8,011	8,552	23,913	25,170
Asia, excluding Japan
Disaggregation of Revenue [Line Items]
Total revenue		3,013	3,387	8,621	9,093
Europe, Middle East and Africa
Disaggregation of Revenue [Line Items]
Total revenue		$ 8,837	$ 2,352	$ 14,830	$ 8,336

[1] Included distribution and service fees of $7.6 million and $7.7 million for the three months ended September 30, 2020 and 2019, respectively, and $22.3 million and $22.1 million for the nine months ended September 30, 2020 and 2019, respectively.

[2] Forward contracts - foreign exchange