Revenue Recognition (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Revenue Streams [Abstract]
Revenue	$ 75,345	$ 85,788	$ 226,088	$ 260,478
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	67,015	75,934	$ 200,893	230,616
Advisory Fees [Member] | Performance-based [Member]
Revenue Streams [Abstract]
Revenue	449	180	807	207
Advisory Fees [Member] | Conditional [Member]
Revenue Streams [Abstract]
Revenue	0	0	$ 0	1,650
Number of days for customer to make payment after being invoiced			60 days
Advisory Fees [Member] | Open-end Funds [Member]
Revenue Streams [Abstract]
Revenue	26,263	31,481	$ 80,215	94,326
Number of days for customer to make payment after being invoiced			30 days
Advisory Fees [Member] | Closed-end Funds [Member]
Revenue Streams [Abstract]
Revenue	16,475	17,337	$ 48,555	51,389
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	843	1,189	$ 2,677	3,430
Number of days for customer to make payment after being invoiced			30 days
Advisory Fees [Member] | Institutional & PWM [Member]
Revenue Streams [Abstract]
Revenue	21,500	24,276	$ 64,421	75,391
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	1,485	1,471	$ 4,218	4,223
Number of days for customer to make payment after being invoiced			30 days
Distribution Fees and Other Income [Member]
Revenue Streams [Abstract]
Revenue	$ 8,330	$ 9,854	$ 25,195	$ 29,862
Number of days for customer to make payment after being invoiced			30 days