License and Collaboration Agreements - Additional Information (Detail)			1 Months Ended								3 Months Ended								12 Months Ended
	Feb. 13, 2018	Aug. 23, 2017 USD ($)	Apr. 30, 2018 USD ($) shares	Jan. 31, 2018 USD ($)	Nov. 30, 2017 USD ($)	Sep. 30, 2017 USD ($)	Aug. 31, 2017 USD ($)	Sep. 30, 2016	Mar. 31, 2016	Oct. 31, 2010 USD ($)	Dec. 31, 2020 USD ($)	Sep. 30, 2020 USD ($)	Jun. 30, 2020 USD ($)	Mar. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2015 USD ($)	Jan. 09, 2020 USD ($)
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Performance obligation recognized																			$ 129,600,000	$ 77,500,000	$ 95,300,000
Potential future additional payments for development milestones											$ 1,700,000,000								1,700,000,000
Accounts receivable											38,889,000				$ 36,802,000				38,889,000	36,802,000
Other long-term liabilities											6,516,000				768,000				6,516,000	768,000
Development milestones received																			5,516,000
Deferred revenue											2,555,000				8,071,000				2,555,000	8,071,000
Revenue recognized from contracts with customers											23,462,000	$ 30,033,000	$ 48,847,000	$ 50,573,000	33,862,000	$ 29,218,000	$ 23,315,000	$ 28,222,000	$ 152,915,000	114,617,000	1,193,323,000
Percentage of net sales to fund external costs incurred, due to reduction of 2 percentage points																			0.02
BMS Collaboration Agreement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Sales milestone revenue																			$ 50,000,000.0
NKTR-358 | Nektar's
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Deferred revenue											0								$ 0
Regulatory milestones payment, description																			A portion of the development milestones may be reduced by 50% under certain conditions, related to the final formulation of the approved product and the timing of prior approval (if any) of competitive products with a similar mechanism of action, which could reduce these milestone payments by 75% if both conditions occur.
Bristol Myers Squibb Collaboration Agreement | Nektar 214 | 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
Bristol Myers Squibb Collaboration Agreement | Nektar 214 | 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
Bristol Myers Squibb Collaboration Agreement | Nektar 214 | Nektar's | Opdivo
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Percentage of sharing development costs	32.50%
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
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Percentage of upfront payment, market access milestones, royalties and sales milestones									40.00%
Eli Lilly | Nektar-358
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Percentage of sharing in Phase 2 development costs		75.00%
Bristol-Myers Squibb | Purchase Agreement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Shares issued (in shares) | shares			8,284,600
Sale of stock consideration received			$ 850,000,000.0
Bristol-Myers Squibb | Nektar 214 | Research and development
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Reimbursement of expenses																			128,200,000	105,400,000	62,500,000
Bristol-Myers Squibb | BMS Collaboration Agreement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Accounts receivable											38,700,000				$ 24,000,000.0				38,700,000	$ 24,000,000.0
Company share of development costs, annual cap																			125,000,000.0
Company share of development costs																			129,000,000.0
Development costs exceeded																			4,000,000.0
Reimbursement recognized																			4,000,000.0
Other long-term liabilities											$ 4,000,000.0								4,000,000.0
Bristol-Myers Squibb | Clinical Trial Agreement | Nektar 214
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Percentage of out-of-pocket costs to be reimbursed by partner								50.00%
Bristol-Myers Squibb | Bristol Myers Squibb 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		1,059,800,000
Potential future development, regulatory and sales milestones																					$ 1,800,000,000
Bristol-Myers Squibb | Bristol Myers Squibb Collaboration Agreement | Nektar 214
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 | Bristol Myers Squibb Collaboration Agreement | Nektar 214 | Opdivo
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Percentage of sharing development costs	67.50%
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 recognized from contracts with customers																		$ 10,000,000.0
Baxalta Incorporated / Takeda | Hemophilia | European Union
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenue recognized from contracts with customers				$ 10,000,000.0
Eli Lilly and Company | Nektar-358
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Amount of transaction price allocated to performance obligation						125,900,000
Revenue recognized from contracts with customers							$ 125,900,000
Eli Lilly and Company | Nektar-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 | Nektar-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 | Nektar-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 | Nektar-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 | Nektar-358
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 | Nektar-358
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Amount of transaction price allocated to performance obligation						$ 6,500,000
Amgen, Inc.
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Received upfront and milestone payment										$ 50,000,000.0
Astra Zeneca Ab | MOVANTIK and MOVENTIG
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Percentage of net sales to fund external costs incurred											0.33								0.33
Amount of net sales to fund external costs incurred cap																			$ 35,000,000.0
Astra Zeneca Ab | MOVANTIK and MOVENTIG | Upfront Payment Arrangement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Received upfront and milestone payment																						$ 37,600,000	$ 385,000,000.0
Astra Zeneca 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
Terminated Partnership
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Deferred revenue											$ 2,000,000.0								$ 2,000,000.0
Milestone One | Bristol Myers Squibb Collaboration Agreement | Nektar 214 | Nektar's
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Potential future additional payments for development milestones																								$ 25,000,000.0