Revenue Recognition (Details) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021 USD ($)	Jun. 30, 2020 USD ($)	Jun. 30, 2021 USD ($) Fund	Jun. 30, 2020 USD ($)
Revenue Streams [Abstract]
Revenue	$ 75,624	$ 57,559	$ 143,552	$ 127,126
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	68,885	51,470	$ 130,355	113,743
Advisory Fees [Member] | Performance-based [Member]
Revenue Streams [Abstract]
Revenue	3,507	248	$ 3,892	343
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	24,709	20,365	$ 48,181	43,921
Number of days for customer to make payment after being invoiced			30 days
Advisory Fees [Member] | Closed-end Funds [Member]
Revenue Streams [Abstract]
Revenue	19,805	14,594	$ 37,887	31,014
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	650	517	$ 1,266	1,249
Number of days for customer to make payment after being invoiced			30 days
Advisory Fees [Member] | Institutional & PWM [Member]
Revenue Streams [Abstract]
Revenue	18,592	14,543	$ 36,191	34,548
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	1,622	1,203	$ 2,938	2,668
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	$ 6,739	$ 6,089	$ 13,197	$ 13,383
Number of days for customer to make payment after being invoiced			30 days