Investments - Schedule of Marketable Securities (Detail) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Available-for-sale:
Cost	$ 83,048	$ 43,956
Unrealized Loss	(3,723)	(2,885)
Unrealized Gain	909	425
Fair Value	80,234	41,496
Sponsored Funds [Member]
Trading:
Cost	69,355	31,167
Unrealized Loss	(3,146)	(2,134)
Unrealized Gain	256	298
Fair Value	66,465	29,331
Equity Securities [Member]
Trading:
Cost	10,217	9,434
Unrealized Loss	(356)	(386)
Unrealized Gain	339	120
Fair Value	10,200	9,168
Sponsored Closed-End Funds [Member]
Available-for-sale:
Cost	3,476	3,355
Unrealized Loss	(221)	(365)
Unrealized Gain	314	7
Fair Value	$ 3,569	$ 2,997