Revenues - Disaggregation of Revenue (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Disaggregation of Revenue [Line Items]
Revenues	$ 132,932	$ 94,132	$ 261,960	$ 173,908
Investment management fees
Disaggregation of Revenue [Line Items]
Revenues	103,168	74,062	203,644	133,333
Open-end funds
Disaggregation of Revenue [Line Items]
Revenues	57,192	39,186	111,566	69,712
Closed-end funds
Disaggregation of Revenue [Line Items]
Revenues	10,168	11,174	20,547	22,253
Retail separate accounts
Disaggregation of Revenue [Line Items]
Revenues	17,091	12,759	33,620	24,192
Institutional accounts
Disaggregation of Revenue [Line Items]
Revenues	15,778	8,790	31,596	13,931
Structured products
Disaggregation of Revenue [Line Items]
Revenues	2,068	1,163	4,394	1,662
Other products
Disaggregation of Revenue [Line Items]
Revenues	871	990	1,921	1,583
Distribution and service fees
Disaggregation of Revenue [Line Items]
Revenues	13,549	10,439	26,156	21,222
Administration and shareholder service fees
Disaggregation of Revenue [Line Items]
Revenues	15,967	9,476	31,705	18,457
Other income and fees
Disaggregation of Revenue [Line Items]
Revenues	$ 248	$ 155	$ 455	$ 896