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Marketable Securities (Narrative) (Details) (USD $)
In Thousands, except Share data, unless otherwise specified
 3 Months Ended 12 Months Ended
Mar. 31, 2015
Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]    
Available-for-sale Securities, Gross Unrealized Loss $ 26us-gaap_AvailableForSaleSecuritiesGrossUnrealizedLoss $ 73us-gaap_AvailableForSaleSecuritiesGrossUnrealizedLoss
Available-For-Sale Securities Number of Securities Owned 35nvax_AvailableForSaleSecuritiesNumberOfSecuritiesOwned  
Available-For-Sale Securities Number Of Securities Owned Unrealized Losses 29nvax_AvailableForSaleSecuritiesNumberOfSecuritiesOwnedUnrealizedLosses  
Available-for-sale Securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Available-for-sale Securities, Gross Unrealized Loss $ 100us-gaap_AvailableForSaleSecuritiesGrossUnrealizedLoss
/ us-gaap_InformationByCategoryOfDebtSecurityAxis
= us-gaap_AvailableforsaleSecuritiesMember