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Stockholders' Equity - Additional Information (Detail) (USD $)
 0 Months Ended 12 Months Ended
Jan. 28, 2014
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Dec. 31, 2011
Class of Stock [Line Items]          
Preferred stock, shares authorized   10,000,000us-gaap_PreferredStockSharesAuthorized 10,000,000us-gaap_PreferredStockSharesAuthorized    
Preferred stock, par value   $ 0.0001us-gaap_PreferredStockParOrStatedValuePerShare $ 0.0001us-gaap_PreferredStockParOrStatedValuePerShare    
Preferred stock, shares designated   0nktr_PreferredStockShareDesignated 0nktr_PreferredStockShareDesignated    
Preferred stock, shares issued   0us-gaap_PreferredStockSharesIssued 0us-gaap_PreferredStockSharesIssued    
Preferred stock, shares outstanding   0us-gaap_PreferredStockSharesOutstanding 0us-gaap_PreferredStockSharesOutstanding    
Proceeds from sale of common stock $ 117,200,000us-gaap_ProceedsFromIssuanceOfCommonStock        
Restricted Stock Units outstanding   0nktr_RestrictedStockUnitsOutstanding      
Common stock, shares authorized   300,000,000us-gaap_CommonStockSharesAuthorized 300,000,000us-gaap_CommonStockSharesAuthorized    
Maximum 401(k) per employee, percentage of annual salary   60.00%nktr_MaximumContributionPerEmployeeAnnualSalaryPercentage      
Compensation expense in connection with 401(k) retirement plan   900,000us-gaap_DefinedContributionPlanCostRecognized 1,000,000us-gaap_DefinedContributionPlanCostRecognized 900,000us-gaap_DefinedContributionPlanCostRecognized  
Severance payment period for executives   12 months      
Severance payment period for employees   6 months      
Cash severance payment for chief executive officer   24 months base salary plus annual target incentive pay      
2012 Performance Incentive Plan [Member]          
Class of Stock [Line Items]          
Additional new shares available for award grants   5,300,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfAdditionalSharesAuthorized
/ us-gaap_AwardTypeAxis
= nktr_TwoThousandTwelveEquityIncentivePlanMember		      
Share-based compensation plan's share limit reduction for every one restricted stock unit granted   1.5nktr_ShareBasedCompensationPlansShareLimitForEveryOneShare
/ us-gaap_AwardTypeAxis
= nktr_TwoThousandTwelveEquityIncentivePlanMember		      
Maximum term of stock options under equity incentive plans   8 years      
2012 Performance Incentive Plan [Member] | Employee [Member]          
Class of Stock [Line Items]          
Service period for stock options granted   4 years      
2012 Performance Incentive Plan [Member] | Director [Member]          
Class of Stock [Line Items]          
Service period for stock options granted   1 year      
2000 Non-Officer Equity Incentive Plan [Member]          
Class of Stock [Line Items]          
Maximum term of stock options under equity incentive plans   8 years      
Restricted Stock Units [Member]          
Class of Stock [Line Items]          
Awards granted   0us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockUnitsRSUMember		 0us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockUnitsRSUMember		 0us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockUnitsRSUMember		  
Employee Stock Purchase Plan [Member]          
Class of Stock [Line Items]          
Common stock, shares authorized   2,500,000us-gaap_CommonStockSharesAuthorized
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockMember		      
Percentage of purchase price of stock   85.00%nktr_PercentageOfPurchasePriceOfStock
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockMember		      
Maximum [Member]          
Class of Stock [Line Items]          
Matching 401(k) employer contribution, maximum amount   3,000us-gaap_DeferredCompensationArrangementWithIndividualEmployerContribution
/ us-gaap_RangeAxis
= us-gaap_MaximumMember		      
Maximum [Member] | 2012 Performance Incentive Plan [Member]          
Class of Stock [Line Items]          
Number of common stock shares available for stock options grants   10,347,140us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant
/ us-gaap_AwardTypeAxis
= nktr_TwoThousandTwelveEquityIncentivePlanMember
/ us-gaap_RangeAxis
= us-gaap_MaximumMember		      
Series A Preferred Stock [Member]          
Class of Stock [Line Items]          
Preferred stock, shares designated   0nktr_PreferredStockShareDesignated
/ us-gaap_StatementClassOfStockAxis
= us-gaap_SeriesAPreferredStockMember		 0nktr_PreferredStockShareDesignated
/ us-gaap_StatementClassOfStockAxis
= us-gaap_SeriesAPreferredStockMember		   3,100,000nktr_PreferredStockShareDesignated
/ us-gaap_StatementClassOfStockAxis
= us-gaap_SeriesAPreferredStockMember
Common Shares [Member]          
Class of Stock [Line Items]          
Common stock, shares issued 9,775,000us-gaap_StockIssuedDuringPeriodSharesNewIssues
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember		 9,775,000us-gaap_StockIssuedDuringPeriodSharesNewIssues
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember		      
Legal, accounting fees, filing fees and other offering expenses 600,000us-gaap_PaymentsOfStockIssuanceCosts
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember		        
Underwriting commission and discounts $ 7,500,000nktr_UnderwritersCommissionsAndDiscount
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember		        
Common stock, shares reserved for issuance in equity compensation plans   24,806,565us-gaap_CommonStockCapitalSharesReservedForFutureIssuance
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember