License and Collaboration Agreements - Additional Information (Detail) - USD ($)			1 Months Ended		3 Months Ended		12 Months Ended
	Feb. 13, 2018	Aug. 23, 2017	Apr. 30, 2018	Sep. 30, 2017	Mar. 31, 2021	Mar. 31, 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					$ 18,800,000	$ 44,600,000
Potential future additional development and regulatory milestones					1,700,000,000
Accounts receivable					29,156,000			$ 38,889,000
BMS Collaboration Agreement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Sales milestone revenue					25,000,000.0
Nektar's | Nektar-358
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's | Other
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Potential future additional payments for development milestones					40,000,000.0
Nektar 214 | Nektar's | Bristol Myers Squibb Collaboration Agreement
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 | Nektar's | Milestone One | Bristol Myers Squibb Collaboration Agreement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Potential development milestones excluded from transaction price					1,800,000,000
Potential future additional payments for development milestones									$ 25,000,000.0
Nektar 214 | Nektar's | Maximum | Bristol Myers Squibb Collaboration Agreement
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 | Nektar's | Opdivo | Bristol Myers Squibb Collaboration Agreement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Percentage of sharing development costs	32.50%
Bristol-Myers Squibb Company | BMS Collaboration Agreement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Accounts receivable					26,200,000
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 Company | Purchase Agreement | Bristol Myers Squibb Collaboration 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
Bristol-Myers Squibb Company | Nektar 214 | Bristol Myers Squibb Collaboration Agreement
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%
Bristol-Myers Squibb Company | Nektar 214 | Research and Development Expense
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Reimbursement of expenses					$ 26,700,000	$ 30,700,000
Bristol-Myers Squibb Company | Nektar 214 | Opdivo | Bristol Myers Squibb Collaboration Agreement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Percentage of sharing development costs	67.50%
Eli Lilly and Company | Maximum | Nektar-358
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Potential future additional development and regulatory milestones		$ 250,000,000.0
Eli Lilly and Company | Nektar's | Nektar-358
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%
Eli Lilly and Company | Nektar's | Maximum | Nektar-358
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%
Eli Lilly and Company | Nektar's | Minimum | Nektar-358
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%
Eli Lilly and Company | Nektar 358 License
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Amount of transaction price allocated to performance obligation				125,900,000
Eli Lilly and Company | Nektar 358 License | Phase 1 Clinical Development
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Amount of transaction price allocated to performance obligation				17,600,000
Eli Lilly and Company | Nektar 358 License | Drug Product Development
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Amount of transaction price allocated to performance obligation				$ 6,500,000
Eli Lilly | Nektar-358
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Percentage of sharing in Phase 2 development costs		75.00%