Revenues - Disaggregation of Revenue (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Disaggregation of Revenue [Line Items]
Revenues	$ 603,896	$ 563,246	$ 552,235
Open-end funds
Disaggregation of Revenue [Line Items]
Revenues	247,519	229,637	231,175
Closed-end funds
Disaggregation of Revenue [Line Items]
Revenues	36,833	42,199	41,455
Retail separate accounts
Disaggregation of Revenue [Line Items]
Revenues	104,932	82,999	73,532
Institutional accounts
Disaggregation of Revenue [Line Items]
Revenues	109,531	96,429	77,711
Structured products
Disaggregation of Revenue [Line Items]
Revenues	4,012	6,381	9,622
Other products
Disaggregation of Revenue [Line Items]
Revenues	2,511	3,832	3,526
Total investment management fees
Disaggregation of Revenue [Line Items]
Revenues	505,338	461,477	437,021
Distribution and service fees
Disaggregation of Revenue [Line Items]
Revenues	38,425	40,898	50,715
Administration and shareholder service fees
Disaggregation of Revenue [Line Items]
Revenues	59,463	59,884	63,614
Other income and fees
Disaggregation of Revenue [Line Items]
Revenues	$ 670	$ 987	$ 885