Marketable Securities - Schedule of Marketable Securities (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Schedule Of Available For Sale And Trading Securities [Line Items]
Cost	$ 63,545	$ 55,037
Unrealized Loss	(3,400)	(891)
Unrealized Gain	3,702	4,129
Fair Value	63,847	58,275
Sponsored mutual funds and variable insurance funds [Member]
Schedule Of Available For Sale And Trading Securities [Line Items]
Cost	11,840	7,312
Unrealized Loss	(779)	(689)
Unrealized Gain	2,310	1,390
Fair Value	13,371	8,013
Investments of consolidated sponsored investment products [Member]
Schedule Of Available For Sale And Trading Securities [Line Items]
Cost	45,138	41,367
Unrealized Loss	(2,536)	(165)
Unrealized Gain	512	2,025
Fair Value	43,114	43,227
Equity securities [Member]
Schedule Of Available For Sale And Trading Securities [Line Items]
Cost	3,852	3,739
Unrealized Gain	797	640
Fair Value	4,649	4,379
Sponsored closed-end funds [Member]
Schedule Of Available For Sale And Trading Securities [Line Items]
Cost	2,715	2,619
Unrealized Loss	(85)	(37)
Unrealized Gain	83	74
Fair Value	$ 2,713	$ 2,656