Investments - Schedule of Marketable Securities (Detail) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Available-for-sale:
Cost	$ 82,616	$ 43,956
Unrealized Loss	(3,786)	(2,885)
Unrealized Gain	675	425
Fair Value	79,505	41,496
Sponsored Funds [Member]
Trading:
Cost	69,144	31,167
Unrealized Loss	(3,186)	(2,134)
Unrealized Gain	186	298
Fair Value	66,144	29,331
Equity Securities [Member]
Trading:
Cost	10,055	9,434
Unrealized Loss	(340)	(386)
Unrealized Gain	333	120
Fair Value	10,048	9,168
Sponsored Closed-End Funds [Member]
Available-for-sale:
Cost	3,417	3,355
Unrealized Loss	(260)	(365)
Unrealized Gain	156	7
Fair Value	$ 3,313	$ 2,997