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Marketable Securities - Schedule of Marketable Securities (Detail) (USD $)
In Thousands, unless otherwise specified
Sep. 30, 2013
Dec. 31, 2012
Schedule Of Available For Sale And Trading Securities [Line Items]    
Fair Value $ 95,998 $ 43,227
Cost 114,784 55,037
Unrealized Loss (3,379) (891)
Unrealized Gain 6,194 4,129
Fair Value 117,599 58,275
Sponsored mutual funds and variable insurance funds [Member]
    
Schedule Of Available For Sale And Trading Securities [Line Items]    
Cost 11,845 7,312
Unrealized Loss (685) (689)
Unrealized Gain 2,653 1,390
Fair Value 13,813 8,013
Debt securities [Member]
    
Schedule Of Available For Sale And Trading Securities [Line Items]    
Cost 29,598 24,830
Unrealized Loss (2,154) (21)
Unrealized Gain 165 1,003
Fair Value 27,609 25,812
Equity securities [Member]
    
Schedule Of Available For Sale And Trading Securities [Line Items]    
Cost 3,898 3,739
Unrealized Gain 1,206 640
Fair Value 5,104 4,379
Cost 66,679 16,537
Unrealized Loss (415) (144)
Unrealized Gain 2,125 1,022
Fair Value 68,389 17,415
Sponsored closed-end funds [Member]
    
Schedule Of Available For Sale And Trading Securities [Line Items]    
Cost 2,764 2,619
Unrealized Loss (125) (37)
Unrealized Gain 45 74
Fair Value $ 2,684 $ 2,656