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Cash and Investments in Marketable Securities - Cash and Investments in Marketable Securities, Including Cash Equivalents and Restricted Cash (Detail) (USD $)
In Thousands, unless otherwise specified
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Jul. 11, 2012
Dec. 31, 2011
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]          
Cash and cash equivalents $ 12,365us-gaap_CashAndCashEquivalentsAtCarryingValue $ 39,067us-gaap_CashAndCashEquivalentsAtCarryingValue $ 25,437us-gaap_CashAndCashEquivalentsAtCarryingValue   $ 15,312us-gaap_CashAndCashEquivalentsAtCarryingValue
Short-term investments 225,459us-gaap_AvailableForSaleSecuritiesCurrent 197,959us-gaap_AvailableForSaleSecuritiesCurrent      
Restricted cash 25,000us-gaap_RestrictedCashAndCashEquivalents 25,000us-gaap_RestrictedCashAndCashEquivalents   25,000us-gaap_RestrictedCashAndCashEquivalents  
Total cash and investments in marketable securities $ 262,824nktr_CashRestrictedCashCashEquivalentsAndAvailableForSaleInvestments $ 262,026nktr_CashRestrictedCashCashEquivalentsAndAvailableForSaleInvestments