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Fair Value Measurements Available-For-Sale Marketable Securities (Details) (USD $)
In Thousands, unless otherwise specified
3 Months Ended 12 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Dec. 31, 2014
Marketable securities classified as available-for-sale      
Amortized Cost $ 81,901us-gaap_AvailableForSaleSecuritiesAmortizedCost   $ 73,949us-gaap_AvailableForSaleSecuritiesAmortizedCost
Unrealized Gains 23,159us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax   19us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
Unrealized Losses (18)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax   (34)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
Estimated Fair Value 105,042us-gaap_AvailableForSaleSecurities   73,934us-gaap_AvailableForSaleSecurities
Realized gains on available-for-sale marketable securities 0us-gaap_AvailableForSaleSecuritiesGrossRealizedGains 0us-gaap_AvailableForSaleSecuritiesGrossRealizedGains  
Realized losses on available-for-sale marketable securities 0us-gaap_AvailableForSaleSecuritiesGrossRealizedLosses 0us-gaap_AvailableForSaleSecuritiesGrossRealizedLosses  
Available For Sale Marketable Securities Maturity Maximum 1 year   1 year
Commercial paper      
Marketable securities classified as available-for-sale      
Amortized Cost 19,734us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CommercialPaperMember
  29,730us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CommercialPaperMember
Unrealized Gains 14us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CommercialPaperMember
  19us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CommercialPaperMember
Estimated Fair Value 19,748us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CommercialPaperMember
  29,749us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CommercialPaperMember
Corporate debt securities      
Marketable securities classified as available-for-sale      
Amortized Cost 48,310us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember
  44,219us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember
Unrealized Losses (18)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember
  (34)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember
Estimated Fair Value 48,292us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember
  44,185us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember
Corporate equity securities      
Marketable securities classified as available-for-sale      
Amortized Cost 13,857us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
   
Unrealized Gains 23,145us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
   
Estimated Fair Value $ 37,002us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember