Investments - Schedule of Marketable Securities (Detail) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Available-for-sale:
Cost	$ 82,413	$ 43,956
Unrealized Loss	(3,034)	(2,885)
Unrealized Gain	2,107	425
Fair Value	81,486	41,496
Sponsored Funds [Member]
Trading:
Cost	68,284	31,167
Unrealized Loss	(2,819)	(2,134)
Unrealized Gain	1,404	298
Fair Value	66,869	29,331
Equity Securities [Member]
Trading:
Cost	10,591	9,434
Unrealized Loss	0	(386)
Unrealized Gain	476	120
Fair Value	11,067	9,168
Sponsored Closed-End Funds [Member]
Available-for-sale:
Cost	3,538	3,355
Unrealized Loss	(215)	(365)
Unrealized Gain	227	7
Fair Value	$ 3,550	$ 2,997