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Organization and Summary of Significant Accounting Policies - Additional Information (Detail)
 9 Months Ended
Sep. 30, 2021
USD ($)
segment
Dec. 31, 2020
USD ($)
Mar. 31, 2020
USD ($)
Organization And Summary Of Significant Accounting Policies [Line Items]      
Cash and investments in marketable securities $ 955,299,000 $ 1,198,558,000  
Number of business segment | segment 1    
Accounts receivable $ 28,994,000 38,889,000  
Allowance for credit loss 0 0  
Aggregate offering price 300,000,000    
NKTR - 181      
Organization And Summary Of Significant Accounting Policies [Line Items]      
Write off of prepayment     $ 19,700,000
Purchase commitment, remaining minimum amount committed     $ 25,500,000
Bristol Myers Squibb Collaboration Agreement      
Organization And Summary Of Significant Accounting Policies [Line Items]      
Accounts receivable 24,900,000 38,700,000  
Collaboration Partner      
Organization And Summary Of Significant Accounting Policies [Line Items]      
Accounts receivable $ 3,100,000 $ 200,000