Investments - Schedule of Marketable Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Marketable securities:
Cost	$ 59,258	$ 63,986
Unrealized Loss	(1,409)	(1,598)
Unrealized Gain	4,481	4,036
Fair Value	62,330	66,424
Sponsored funds
Marketable securities:
Cost	39,568	47,084
Unrealized Loss	(983)	(1,294)
Unrealized Gain	1,366	1,059
Fair Value	39,951	46,849
Fair Value	39,951	46,849
Equity securities
Marketable securities:
Cost	15,903	13,141
Unrealized Loss	(9)	(2)
Unrealized Gain	2,747	2,671
Fair Value	18,641	15,810
Fair Value	18,641	15,810
Sponsored closed-end funds
Marketable securities:
Cost	3,787	3,761
Unrealized Loss	(417)	(302)
Unrealized Gain	368	306
Fair Value	3,738	3,765
Fair Value	$ 3,738	$ 3,765