License and Collaboration Agreements - Additional Information (Detail) - USD ($)			1 Months Ended		3 Months Ended		6 Months Ended		12 Months Ended
	Feb. 13, 2018	Aug. 23, 2017	Apr. 30, 2018	Sep. 30, 2017	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020	Dec. 31, 2018	Dec. 31, 2020	Jan. 09, 2020
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenue recognized in period related to performance in prior periods					$ 20,500,000	$ 42,100,000	$ 39,300,000	$ 86,700,000
Potential future additional development and regulatory milestones (up to)					1,700,000,000		1,700,000,000
Accounts receivable					28,871,000		28,871,000			$ 38,889,000
Bristol-Myers Squibb Company | Purchase Agreement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Shares issued (in shares)			8,284,600
Sale of stock consideration received			$ 850,000,000.0
Bristol Myers Squibb Collaboration Agreement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Sales milestone, revenue						25,000,000.0		50,000,000.0
Bristol Myers Squibb Collaboration Agreement | Bristol-Myers Squibb Company | Purchase Agreement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total consideration received under agreements									$ 1,850,000,000
Estimated fair value of shares									790,200,000
Remaining amount allocated to transaction price									$ 1,059,800,000
Nektar-358 | Nektar's
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Received upfront and milestone payment				$ 150,000,000.0
Percentage of sharing in Phase 2 development costs		25.00%
Nektar-358 | Eli Lilly and Company | Maximum
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Potential future additional development and regulatory milestones (up to)		$ 250,000,000.0
Nektar-358 | Eli Lilly and Company | Nektar's
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Percentage of regulatory milestones payments will be reduced under certain conditions		50.00%
Percentage of regulatory milestones payments will be reduced if conditions occur		75.00%
Nektar-358 | Eli Lilly and Company | Nektar's | Maximum
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Percentage of funding phase 3 development costs on an indication by indication basis borne		25.00%
Nektar-358 | Eli Lilly and Company | Nektar's | Minimum
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Percentage of funding phase 3 development costs on an indication by indication basis borne		0.00%
Nektar-358 | Eli Lilly
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Percentage of sharing in Phase 2 development costs		75.00%
Other | Nektar's
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Potential future additional payments for development milestones (up to)					40,000,000.0		40,000,000.0
Nektar 214 | Bristol-Myers Squibb Company | Research and Development Expense
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Reimbursement of expenses					24,800,000	$ 33,900,000	51,400,000	$ 65,100,000
Nektar 214 | Bristol Myers Squibb Collaboration Agreement | Nektar's
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Global commercialization profits and losses sharing percentage	65.00%
Percentage of sharing production costs	65.00%
Upfront and milestone payments received from license agreements			1,000,000,000.0
Maximum total additional cash payments receivable upon achievement of certain development and regulatory milestones							1,455,000,000
Nektar 214 | Bristol Myers Squibb Collaboration Agreement | Nektar's | Milestone One
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Potential future additional payments for development milestones (up to)											$ 25,000,000.0
Potential development milestones excluded from transaction price (up to)					$ 1,800,000,000		$ 1,800,000,000
Nektar 214 | Bristol Myers Squibb Collaboration Agreement | Nektar's | Maximum
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Eligible additional cash payments receivable upon achievement of certain sales milestones			$ 350,000,000.0
Nektar 214 | Bristol Myers Squibb Collaboration Agreement | Nektar's | Opdivo
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Percentage of sharing development costs	32.50%
Nektar 214 | Bristol Myers Squibb Collaboration Agreement | Bristol-Myers Squibb Company
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Global commercialization profits and losses sharing percentage	35.00%
Percentage of sharing production costs	35.00%
Nektar 214 | Bristol Myers Squibb Collaboration Agreement | Bristol-Myers Squibb Company | Opdivo
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Percentage of sharing development costs	67.50%
Nektar 358 License | Eli Lilly and Company
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Amount of transaction price allocated to performance obligation				125,900,000
Nektar 358 License | Phase 1 Clinical Development | Eli Lilly and Company
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Amount of transaction price allocated to performance obligation				17,600,000
Nektar 358 License | Drug Product Development | Eli Lilly and Company
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Amount of transaction price allocated to performance obligation				$ 6,500,000