Investments - Schedule of Marketable Securities (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Marketable Securities [Abstract]
Cost	$ 58,854	$ 63,986
Unrealized Loss	(1,321)	(1,598)
Unrealized Gain	4,506	4,036
Fair Value	62,039	66,424
Sponsored funds
Marketable Securities [Abstract]
Cost	41,463	47,084
Unrealized Loss	(928)	(1,294)
Unrealized Gain	1,380	1,059
Fair Value	41,915	46,849
Equity securities
Marketable Securities [Abstract]
Cost	13,621	13,141
Unrealized Loss	(11)	(2)
Unrealized Gain	2,939	2,671
Fair Value	16,549	15,810
Sponsored closed-end funds
Marketable Securities [Abstract]
Cost	3,770	3,761
Unrealized Loss	(382)	(302)
Unrealized Gain	187	306
Fair Value	$ 3,575	$ 3,765