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Cash, Cash Equivalents and Marketable Securities (Details 1) (USD $)
In Thousands, unless otherwise specified
Jun. 30, 2012
Dec. 31, 2011
Assets measured at fair value on a recurring basis    
Assets measured at fair value $ 93,942 $ 98,131
Cash and Money Market Fund [Member]
    
Assets measured at fair value on a recurring basis    
Assets measured at fair value 28,886 62,446
Commercial Paper [Member]
    
Assets measured at fair value on a recurring basis    
Assets measured at fair value 54,932 10,698
Corporate securities [Member]
    
Assets measured at fair value on a recurring basis    
Assets measured at fair value 10,124 24,987
Level 1 [Member]
    
Assets measured at fair value on a recurring basis    
Assets measured at fair value 28,886 62,446
Level 1 [Member] | Cash and Money Market Fund [Member]
    
Assets measured at fair value on a recurring basis    
Assets measured at fair value 28,886 62,446
Level 2 [Member]
    
Assets measured at fair value on a recurring basis    
Assets measured at fair value 65,056 35,685
Level 2 [Member] | Commercial Paper [Member]
    
Assets measured at fair value on a recurring basis    
Assets measured at fair value 54,932 10,698
Level 2 [Member] | Corporate securities [Member]
    
Assets measured at fair value on a recurring basis    
Assets measured at fair value 10,124 24,987
Level 3 [Member]
    
Assets measured at fair value on a recurring basis    
Assets measured at fair value      
Level 3 [Member] | Cash and Money Market Fund [Member]
    
Assets measured at fair value on a recurring basis    
Assets measured at fair value      
Level 3 [Member] | Commercial Paper [Member]
    
Assets measured at fair value on a recurring basis    
Assets measured at fair value      
Level 3 [Member] | Corporate securities [Member]
    
Assets measured at fair value on a recurring basis    
Assets measured at fair value