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Marketable Securities (Investments Available For Sale) (Details) (USD $)
In Thousands, unless otherwise specified
Mar. 31, 2013
Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]    
Investment Available for Sale, Cost Amount $ 68,172 $ 71,486
Investment Available for Sale, Gross Unrealized Gains 4,147 4,396
Investment Available for Sale, Gross Unrealized Losses (24) (13)
Investment Available for Sale, Fair Value 72,295 75,869
SEI-Sponsored Mutual Funds [Member]
   
Schedule of Available-for-sale Securities [Line Items]    
Investment Available for Sale, Cost Amount 9,153 12,953
Investment Available for Sale, Gross Unrealized Gains 817 376
Investment Available for Sale, Gross Unrealized Losses (9) (13)
Investment Available for Sale, Fair Value 9,961 13,316
Equity Securities [Member]
   
Schedule of Available-for-sale Securities [Line Items]    
Investment Available for Sale, Cost Amount 2,600 2,610
Investment Available for Sale, Gross Unrealized Gains 0 0
Investment Available for Sale, Gross Unrealized Losses (15) 0
Investment Available for Sale, Fair Value 2,585 2,610
Debt Securities [Member]
   
Schedule of Available-for-sale Securities [Line Items]    
Investment Available for Sale, Cost Amount 56,419 55,923
Investment Available for Sale, Gross Unrealized Gains 3,330 4,020
Investment Available for Sale, Gross Unrealized Losses 0 0
Investment Available for Sale, Fair Value $ 59,749 $ 59,943