Revenue Recognition (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Revenue Streams [Abstract]
Revenue	$ 85,788	$ 88,341	$ 260,478	$ 261,858
Closed-end Funds [Member] | Performance Correlated [Member]
Revenue Streams [Abstract]
Number of days for customer to make payment after being invoiced			30 days
Advisory Fees [Member]
Revenue Streams [Abstract]
Revenue	75,934	77,328	$ 230,616	228,942
Advisory Fees [Member] | Performance Correlated [Member]
Revenue Streams [Abstract]
Revenue	180	10	207	24
Advisory Fees [Member] | Contingent [Member]
Revenue Streams [Abstract]
Revenue	0	0	$ 1,650	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	31,481	32,738	$ 94,326	97,390
Number of days for customer to make payment after being invoiced			30 days
Advisory Fees [Member] | Closed-end Funds [Member]
Revenue Streams [Abstract]
Revenue	17,337	16,242	$ 51,389	47,404
Number of days for customer to make payment after being invoiced			30 days
Advisory Fees [Member] | Closed-end Funds [Member] | Performance Correlated [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	1,189	1,024	$ 3,430	2,321
Number of days for customer to make payment after being invoiced			30 days
Advisory Fees [Member] | Institutional and Private Wealth Management Clients [Member]
Revenue Streams [Abstract]
Revenue	24,276	26,049	$ 75,391	78,603
Number of days for customer to make payment after being invoiced			60 days
Advisory Fees [Member] | Institutional and Private Wealth Management Clients [Member] | Performance Correlated [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	1,471	1,265	$ 4,223	3,200
Number of days for customer to make payment after being invoiced			30 days
Distribution Fees [Member]
Revenue Streams [Abstract]
Revenue	$ 9,854	$ 11,013	$ 29,862	$ 32,916
Number of days for customer to make payment after being invoiced			30 days