Revenues - Disaggregation of Revenue (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Disaggregation of Revenue [Line Items]
Revenues	$ 129,028	$ 79,776
Investment management fees
Disaggregation of Revenue [Line Items]
Revenues	100,476	59,271
Open-end funds
Disaggregation of Revenue [Line Items]
Revenues	54,361	30,526
Closed-end funds
Disaggregation of Revenue [Line Items]
Revenues	10,378	11,079
Retail separate accounts
Disaggregation of Revenue [Line Items]
Revenues	16,529	11,434
Institutional accounts
Disaggregation of Revenue [Line Items]
Revenues	15,818	5,141
Structured products
Disaggregation of Revenue [Line Items]
Revenues	2,326	498
Other products
Disaggregation of Revenue [Line Items]
Revenues	1,064	593
Distribution and service fees
Disaggregation of Revenue [Line Items]
Revenues	12,607	10,783
Administration and shareholder service fees
Disaggregation of Revenue [Line Items]
Revenues	15,738	8,981
Other income and fees
Disaggregation of Revenue [Line Items]
Revenues	$ 207	$ 741