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Financial Instruments - Cash, Cash Equivalents and Marketable Securities (Details) (US Treasury bills and notes)
6 Months Ended
Jun. 30, 2012
Minimum
 
Financial Instruments - Cash, Cash Equivalents and Marketable Securities  
Maturity period of long-term marketable securities 1 year
Maximum
 
Financial Instruments - Cash, Cash Equivalents and Marketable Securities  
Maturity period of long-term marketable securities 2 years