Investments - Schedule of Marketable Securities (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Marketable securities:
Cost	$ 58,031	$ 63,986
Unrealized Loss	1,265	1,598
Unrealized Gain	4,635	4,036
Fair Value	61,401	66,424
Sponsored funds
Marketable securities:
Cost	40,235	47,084
Unrealized Loss	(895)	(1,294)
Unrealized Gain	1,333	1,059
Fair Value	40,673	46,849
Fair Value	40,673	46,849
Equity securities
Marketable securities:
Cost	14,018	13,141
Unrealized Loss	10	2
Unrealized Gain	3,000	2,671
Fair Value	17,008	15,810
Fair Value	17,008	15,810
Sponsored closed-end funds
Marketable securities:
Cost	3,778	3,761
Unrealized Loss	360	302
Unrealized Gain	302	306
Fair Value	3,720	3,765
Fair Value	$ 3,720	$ 3,765