Revenues - Disaggregation of Revenue (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Disaggregation of Revenue [Line Items]
Revenues	$ 140,489	$ 132,932	$ 271,207	$ 261,960
Investment management fees
Disaggregation of Revenue [Line Items]
Revenues	114,591	103,168	220,509	203,644
Open-end funds
Disaggregation of Revenue [Line Items]
Revenues	56,973	57,192	110,266	111,566
Closed-end funds
Disaggregation of Revenue [Line Items]
Revenues	10,620	10,168	20,639	20,547
Retail separate accounts
Disaggregation of Revenue [Line Items]
Revenues	20,664	17,091	38,669	33,620
Institutional accounts
Disaggregation of Revenue [Line Items]
Revenues	23,656	15,778	45,833	31,596
Structured products
Disaggregation of Revenue [Line Items]
Revenues	1,585	2,068	3,232	4,394
Other products
Disaggregation of Revenue [Line Items]
Revenues	1,093	871	1,870	1,921
Distribution and service fees
Disaggregation of Revenue [Line Items]
Revenues	10,617	13,549	20,680	26,156
Administration and shareholder service fees
Disaggregation of Revenue [Line Items]
Revenues	15,054	15,967	29,467	31,705
Other income and fees
Disaggregation of Revenue [Line Items]
Revenues	$ 227	$ 248	$ 551	$ 455