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Short-Term Investments (Details) (Debt Securities, USD $)
In Millions, unless otherwise specified
3 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Dec. 31, 2014
Schedule of Available-for-sale Securities      
Available-for-sale debt securities, gross unrealized loss $ (0.02)us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax   $ (0.03)us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
Available-for-sale debt securities, gross unrealized gain 0.01us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax    
Contractual maturity 2 years    
Available-for-sale debt securities, gross realized gain   0.03us-gaap_AvailableForSaleSecuritiesGrossRealizedGains  
Available-for-sale debt securities, gross realized loss   (0.5)us-gaap_AvailableForSaleSecuritiesGrossRealizedLosses  
Reclassification out of Accumulated Other Comprehensive Income      
Schedule of Available-for-sale Securities      
Available-for-sale debt securities, gross realized gain   0.03us-gaap_AvailableForSaleSecuritiesGrossRealizedGains
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
/ us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis
= us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeMember
 
Available-for-sale debt securities, gross realized loss   $ (0.5)us-gaap_AvailableForSaleSecuritiesGrossRealizedLosses
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
/ us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis
= us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeMember