Revenues - Disaggregation of Revenue (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Disaggregation of Revenue [Line Items]
Revenues	$ 152,210	$ 123,675	$ 414,170	$ 297,583
Investment management fees
Disaggregation of Revenue [Line Items]
Revenues	121,713	97,295	325,357	230,628
Open-end funds
Disaggregation of Revenue [Line Items]
Revenues	62,466	51,190	174,032	120,902
Closed-end funds
Disaggregation of Revenue [Line Items]
Revenues	10,614	11,243	31,161	33,495
Retail separate accounts
Disaggregation of Revenue [Line Items]
Revenues	19,532	14,686	53,152	38,879
Institutional accounts
Disaggregation of Revenue [Line Items]
Revenues	24,614	16,208	56,210	30,140
Structured products
Disaggregation of Revenue [Line Items]
Revenues	3,602	2,822	7,996	4,483
Other products
Disaggregation of Revenue [Line Items]
Revenues	885	1,146	2,806	2,729
Distribution and service fees
Disaggregation of Revenue [Line Items]
Revenues	13,730	11,482	39,886	32,704
Administration and shareholder service fees
Disaggregation of Revenue [Line Items]
Revenues	16,567	14,699	48,272	33,156
Other income and fees
Disaggregation of Revenue [Line Items]
Revenues	$ 200	$ 199	$ 655	$ 1,095