Investments - Schedule of Marketable Securities (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Available-for-sale:
Cost	$ 63,986	$ 75,862
Unrealized Loss	(1,598)	(2,207)
Unrealized Gain	4,036	1,252
Fair Value	66,424	74,907
Sponsored funds
Trading:
Cost	47,084	61,784
Unrealized Loss	(1,294)	(1,942)
Unrealized Gain	1,059	177
Fair Value	46,849	60,019
Equity securities
Trading:
Cost	13,141	10,578
Unrealized Loss	(2)	0
Unrealized Gain	2,671	895
Fair Value	15,810	11,473
Sponsored closed-end funds
Available-for-sale:
Cost	3,761	3,500
Unrealized Loss	(302)	(265)
Unrealized Gain	306	180
Fair Value	$ 3,765	$ 3,415

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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