Organization and Summary of Significant Accounting Policies (Detail) $ in Thousands, shares in Millions		12 Months Ended
	Apr. 13, 2020 USD ($)	Dec. 31, 2020 USD ($) reporting_unit shares	Dec. 31, 2019 USD ($) shares	Dec. 31, 2018 shares	Mar. 31, 2020 USD ($)
Organization And Summary Of Significant Accounting Policies [Line Items]
Cash and investments in marketable securities		$ 1,198,558	$ 1,603,981
Net expense reimbursements from collaboration partner		38,700	24,000
Contract with customer, asset, reclassified to receivable		$ 200	12,800
Estimate for royalties resulting from net sales			$ 11,600
Number of reporting unit evaluated for goodwill | reporting_unit		1
Purchase commitments related to contract manufacturing, clinical development and certain other items		$ 8,000
Weighted-average of potentially dilutive securities (stock options and RSUs) included in calculation of diluted securities under treasury stock method (in shares) | shares				10.5
Accounts Receivable | Customer Concentration Risk | Partner One
Organization And Summary Of Significant Accounting Policies [Line Items]
Percentage revenue from customers			65.00%
Accounts Receivable | Customer Concentration Risk | Partner Two
Organization And Summary Of Significant Accounting Policies [Line Items]
Percentage revenue from customers			17.00%
Accounts Receivable | Customer Concentration Risk | Partner Three
Organization And Summary Of Significant Accounting Policies [Line Items]
Percentage revenue from customers			14.00%
NKTR - 181
Organization And Summary Of Significant Accounting Policies [Line Items]
Write off of prepayment					$ 19,700
Purchase commitments related to contract manufacturing, clinical development and certain other items					$ 25,500
7.75 Percent Senior Secured Notes Due October 2020 | Senior Notes
Organization And Summary Of Significant Accounting Policies [Line Items]
Principal and interest repaid on senior notes	$ 254,800
Shares of Common Stock Underlying Outstanding Stock Options
Organization And Summary Of Significant Accounting Policies [Line Items]
Weighted average antidilutive securities excluded from computation of earnings per share | shares				3.3
Stock Options and Restricted Stock Units
Organization And Summary Of Significant Accounting Policies [Line Items]
Weighted average antidilutive securities excluded from computation of earnings per share | shares		17.4	17.9
Manufacturing equipment | Minimum
Organization And Summary Of Significant Accounting Policies [Line Items]
Property and equipment estimated useful lives		3 years
Manufacturing equipment | Maximum
Organization And Summary Of Significant Accounting Policies [Line Items]
Property and equipment estimated useful lives		10 years
Buildings
Organization And Summary Of Significant Accounting Policies [Line Items]
Property and equipment estimated useful lives		20 years