License and Collaboration Agreements - Additional Information (Detail) - USD ($)			1 Months Ended						3 Months Ended					6 Months Ended	9 Months Ended		12 Months Ended	76 Months Ended
	Feb. 13, 2018	Aug. 23, 2017	Apr. 30, 2018	Jan. 31, 2018	Nov. 30, 2017	Sep. 30, 2017	Aug. 31, 2017	Mar. 31, 2016	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2019	Dec. 31, 2010	Jun. 30, 2020	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2018	Dec. 31, 2015	Jan. 09, 2020	Dec. 31, 2019
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenue recognized in period related to performance in prior periods									$ 22,700,000						$ 109,400,000
Potential future additional development and regulatory milestones									1,700,000,000						1,700,000,000
Accounts receivable									42,925,000						42,925,000					$ 36,802,000
Revenue, excluding assessed tax									30,033,000			$ 29,218,000			129,453,000	$ 80,755,000
Deferred revenue									3,001,000						3,001,000					$ 8,071,000
BMS Collaboration Agreement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Sales milestone, revenue															50,000,000.0
BMS Collaboration Agreement | Bempegaldesleukin | 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
BMS Collaboration Agreement | Bempegaldesleukin | Nektar's | Milestone One
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Potential future additional payments for development milestones																			$ 25,000,000.0
BMS Collaboration Agreement | Bempegaldesleukin | 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
BMS Collaboration Agreement | Bempegaldesleukin | Nektar's | Opdivo
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Percentage of sharing development costs	32.50%
NKTR-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
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Percentage of upfront payment, market access milestones, royalties and sales milestones								40.00%
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 | Bempegaldesleukin | Research and Development Expense
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Reimbursement of expenses									29,200,000			$ 28,300,000			93,800,000	$ 81,400,000
Bristol-Myers Squibb Company | BMS Collaboration Agreement | 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										$ 25,000,000.0	$ 25,000,000.0			$ 50,000,000.0			1,059,800,000
Potential future development, regulatory and sales milestones																	1,800,000,000
Bristol-Myers Squibb Company | BMS Collaboration Agreement | Bempegaldesleukin
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 | BMS Collaboration Agreement | Bempegaldesleukin | Opdivo
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Percentage of sharing development costs	67.50%
Eli Lilly and Company | NKTR-358
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenue, excluding assessed tax							$ 125,900,000
Eli Lilly and Company | NKTR-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 | NKTR-358 | Maximum
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Potential future additional development and regulatory milestones		$ 250,000,000.0
Eli Lilly and Company | NKTR-358 | 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%
Eli Lilly and Company | NKTR-358 | 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%
Eli Lilly and Company | NKTR-358 | 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%
Eli Lilly and Company | Phase 1 Clinical Development | NKTR-358 License
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Amount of transaction price allocated to performance obligation						17,600,000
Eli Lilly and Company | Drug Product Development | NKTR-358 License
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Amount of transaction price allocated to performance obligation						$ 6,500,000
Eli Lilly | NKTR-358
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Percentage of sharing in Phase 2 development costs		75.00%
Baxalta Incorporated/Takeda | Hemophilia
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Received under right to sublicense agreement					$ 12,000,000.0
Baxalta Incorporated/Takeda | Hemophilia | Annual Sales Level Milestone
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenue, excluding assessed tax																	$ 10,000,000.0
Baxalta Incorporated/Takeda | Hemophilia | European Union
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenue, excluding assessed tax				$ 10,000,000.0
Amgen, Inc.
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Received upfront and milestone payment													$ 50,000,000.0
Deferred revenue									400,000						400,000
AstraZeneca AB | MOVANTIK and MOVENTIG
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Received upfront and milestone payment																		$ 385,000,000.0
AstraZeneca AB | AstraZeneca-Kirin
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Deferred revenue									0						0
Percentage of upfront payment, market access milestone payments, royalties and sales milestone payments from sublicense agreement retained by our collaboration partner								60.00%
Other
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Potential future additional payments for development milestones									40,000,000.0						40,000,000.0
Deferred revenue									$ 2,000,000.0						$ 2,000,000.0