Revenue Recognition (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Revenue Streams [Abstract]
Revenue	$ 69,567	$ 74,336
Closed-end Funds [Member] | Performance-based [Member]
Revenue Streams [Abstract]
Number of days for customer to make payment after being invoiced	30 days
Advisory Fees [Member]
Revenue Streams [Abstract]
Revenue	$ 62,273	65,888
Advisory Fees [Member] | Performance-based [Member]
Revenue Streams [Abstract]
Revenue	95	178
Advisory Fees [Member] | Conditional [Member]
Revenue Streams [Abstract]
Revenue	$ 0	0
Number of days for customer to make payment after being invoiced	60 days
Advisory Fees [Member] | Open-end Funds [Member]
Revenue Streams [Abstract]
Revenue	$ 23,556	26,925
Number of days for customer to make payment after being invoiced	30 days
Advisory Fees [Member] | Closed-end Funds [Member]
Revenue Streams [Abstract]
Revenue	$ 16,420	15,789
Number of days for customer to make payment after being invoiced	30 days
Advisory Fees [Member] | Closed-end Funds [Member] | Performance-based [Member]
Revenue Streams [Abstract]
Number of days for customer to make payment after being invoiced	30 days
Advisory Fees [Member] | Sub-advisory Accounts [Member]
Revenue Streams [Abstract]
Revenue	$ 732	935
Number of days for customer to make payment after being invoiced	30 days
Advisory Fees [Member] | Institutional & PWM [Member]
Revenue Streams [Abstract]
Revenue	$ 20,005	20,726
Number of days for customer to make payment after being invoiced	60 days
Advisory Fees [Member] | Institutional & PWM [Member] | Performance-based [Member]
Revenue Streams [Abstract]
Number of days for customer to make payment after being invoiced	60 days
Advisory Fees [Member] | SICAVs [Member]
Revenue Streams [Abstract]
Revenue	$ 1,465	1,335
Number of days for customer to make payment after being invoiced	30 days
Distribution Fees and Other Income [Member]
Revenue Streams [Abstract]
Revenue	$ 7,294	$ 8,448
Number of days for customer to make payment after being invoiced	30 days