Revenues - Disaggregation of Revenue (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Disaggregation of Revenue [Line Items]
Revenues	$ 563,246	$ 552,235	$ 425,607
Open-end funds
Disaggregation of Revenue [Line Items]
Revenues	229,637	231,175	175,260
Closed-end funds
Disaggregation of Revenue [Line Items]
Revenues	42,199	41,455	44,687
Retail separate accounts
Disaggregation of Revenue [Line Items]
Revenues	82,999	73,532	54,252
Institutional accounts
Disaggregation of Revenue [Line Items]
Revenues	96,429	77,711	46,600
Structured products
Disaggregation of Revenue [Line Items]
Revenues	6,381	9,622	6,302
Other products
Disaggregation of Revenue [Line Items]
Revenues	3,832	3,526	3,974
Total investment management fees
Disaggregation of Revenue [Line Items]
Revenues	461,477	437,021	331,075
Distribution and service fees
Disaggregation of Revenue [Line Items]
Revenues	40,898	50,715	44,322
Administration and shareholder service fees
Disaggregation of Revenue [Line Items]
Revenues	59,884	63,614	48,996
Other income and fees
Disaggregation of Revenue [Line Items]
Revenues	$ 987	$ 885	$ 1,214