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Marketable Securities (Narrative) (Details) (USD $)
In Thousands, unless otherwise specified
12 Months Ended
Dec. 31, 2011
Dec. 31, 2010
Dec. 31, 2009
Marketable Securities [Abstract]      
Interest receivable $ 16,352 $ 15,324  
Other-than-temporary impairment, total amount related to auction rate securities loss 16,500   3,134
Net gain (impairment net of gains) on sale of marketable securities previously impaired (2,017) 785 1,277
Net gain on sale of marketable securities previously impaired     $ 1,857