Revenue Recognition (Details) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020 USD ($)	Jun. 30, 2019 USD ($)	Jun. 30, 2020 USD ($) Fund	Jun. 30, 2019 USD ($)
Revenue Streams [Abstract]
Revenue	$ 57,559	$ 76,407	$ 127,126	$ 150,743
Performance-based [Member] | Minimum [Member]
Revenue Streams [Abstract]
Revenue			$ 0
Closed-end Funds [Member] | Performance-based [Member]
Revenue Streams [Abstract]
Number of days for customer to make payment after being invoiced			30 days
Number of funds | Fund			2
Advisory Fees [Member]
Revenue Streams [Abstract]
Revenue	51,470	67,990	$ 113,743	133,878
Advisory Fees [Member] | Performance-based [Member]
Revenue Streams [Abstract]
Revenue	248	180	$ 343	358
Advisory Fees [Member] | Conditional [Member]
Revenue Streams [Abstract]
Number of days for customer to make payment after being invoiced			60 days
Advisory Fees [Member] | Mutual Funds [Member]
Revenue Streams [Abstract]
Revenue	20,365	27,027	$ 43,921	53,952
Number of days for customer to make payment after being invoiced			30 days
Advisory Fees [Member] | Closed-end Funds [Member]
Revenue Streams [Abstract]
Revenue	14,594	16,291	$ 31,014	32,080
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	517	899	$ 1,249	1,834
Number of days for customer to make payment after being invoiced			30 days
Advisory Fees [Member] | Institutional & PWM [Member]
Revenue Streams [Abstract]
Revenue	14,543	22,195	$ 34,548	42,921
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	1,203	1,398	$ 2,668	2,733
Number of days for customer to make payment after being invoiced			30 days
Advisory Fees [Member] | SICAVs [Member] | Performance-based [Member] | Non-US Closed-end Fund [Member]
Revenue Streams [Abstract]
Number of funds | Fund			1
Distribution Fees and Other Income [Member]
Revenue Streams [Abstract]
Revenue	$ 6,089	$ 8,417	$ 13,383	$ 16,865
Number of days for customer to make payment after being invoiced			30 days