Investments - Schedule of Marketable Securities (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Available-for-sale:
Cost	$ 75,862	$ 43,956
Unrealized Loss	(2,207)	(2,885)
Unrealized Gain	1,252	425
Fair Value	74,907	41,496
Sponsored funds [Member]
Trading:
Cost	61,784	31,167
Unrealized Loss	(1,942)	(2,134)
Unrealized Gain	177	298
Fair Value	60,019	29,331
Equity securities [Member]
Trading:
Cost	10,578	9,434
Unrealized Loss	0	(386)
Unrealized Gain	895	120
Fair Value	11,473	9,168
Sponsored closed-end funds [Member]
Available-for-sale:
Cost	3,500	3,355
Unrealized Loss	(265)	(365)
Unrealized Gain	180	7
Fair Value	$ 3,415	$ 2,997