Revenues - Disaggregation of Revenue (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Disaggregation of Revenue [Line Items]
Revenues	$ 552,235	$ 425,607	$ 322,554
Total investment management fees
Disaggregation of Revenue [Line Items]
Revenues	437,021	331,075	235,230
Open-end funds
Disaggregation of Revenue [Line Items]
Revenues	231,175	175,260	129,542
Closed-end funds
Disaggregation of Revenue [Line Items]
Revenues	41,455	44,687	43,342
Retail separate accounts
Disaggregation of Revenue [Line Items]
Revenues	73,532	54,252	40,155
Institutional accounts
Disaggregation of Revenue [Line Items]
Revenues	77,711	46,600	18,707
Structured products
Disaggregation of Revenue [Line Items]
Revenues	9,622	6,302	2,211
Other products
Disaggregation of Revenue [Line Items]
Revenues	3,526	3,974	1,273
Distribution and service fees
Disaggregation of Revenue [Line Items]
Revenues	50,715	44,322	48,250
Administration and shareholder service fees
Disaggregation of Revenue [Line Items]
Revenues	63,614	48,996	38,261
Other income and fees
Disaggregation of Revenue [Line Items]
Revenues	$ 885	$ 1,214	$ 813