Revenue Recognition (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2019 USD ($) Fund	Dec. 31, 2018 USD ($)
Revenue Streams [Abstract]
Number of closed-end funds | Fund	16
Revenue	$ 312,368	$ 341,455
Closed-end Funds [Member]
Revenue Streams [Abstract]
Number of closed-end funds | Fund	2
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	$ 279,090	302,651
Advisory Fees [Member] | Performance-based [Member]
Revenue Streams [Abstract]
Revenue	14,496	1,495
Advisory Fees [Member] | Conditional [Member]
Revenue Streams [Abstract]
Revenue	0	1,650
Advisory Fees [Member] | Open-end Funds [Member]
Revenue Streams [Abstract]
Revenue	$ 105,975	122,804
Number of days for customer to make payment after being invoiced	30 days
Advisory Fees [Member] | Closed-end Funds [Member]
Revenue Streams [Abstract]
Revenue	$ 65,569	67,418
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	$ 3,494	4,479
Number of days for customer to make payment after being invoiced	30 days
Advisory Fees [Member] | Institutional & PWM [Member]
Revenue Streams [Abstract]
Revenue	$ 83,814	99,180
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	$ 5,742	5,625
Number of days for customer to make payment after being invoiced	30 days
Advisory Fees [Member] | SICAVs [Member] | Performance-based [Member]
Revenue Streams [Abstract]
Number of days for customer to make payment after being invoiced	30 days
Distribution Fees and Other Income [Member]
Revenue Streams [Abstract]
Revenue	$ 33,278	$ 38,804
Number of days for customer to make payment after being invoiced	30 days