License and Collaboration Agreements - Additional Information (Detail) - USD ($)				1 Months Ended								3 Months Ended									12 Months Ended
	Feb. 13, 2018	Jan. 01, 2018	Aug. 23, 2017	Apr. 30, 2018	Oct. 31, 2017	Sep. 30, 2017	Jan. 31, 2017	Sep. 30, 2016	Mar. 31, 2016	Jun. 30, 2014	Oct. 31, 2010	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2015	Dec. 31, 2013	Dec. 31, 2007	Jan. 09, 2020
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Performance obligation recognized																					$ 77,500,000	$ 95,300,000
Potential future additional payments for development milestones												$ 1,700,000,000									1,700,000,000
Deferred revenue, current portion												5,517,000				$ 13,892,000					5,517,000	13,892,000
Deferred revenue, less current portion												2,554,000				10,744,000					2,554,000	10,744,000
Upfront and milestone payments received from license agreements																					10,000,000.0
Accounts receivable												36,802,000				43,213,000					36,802,000	43,213,000
Development milestones received																					16,565,000
Deferred revenue												8,071,000				24,636,000					8,071,000	24,636,000
Revenue recognized from contracts with customers												33,862,000	$ 29,218,000	$ 23,315,000	$ 28,222,000	39,826,000	$ 27,762,000	$ 1,087,717,000	$ 38,018,000		114,617,000	1,193,323,000	$ 307,711,000
bempegaldesleukin | Nektar's
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
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				1,400,000,000
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
ADYNOVATE
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Sales milestone revenue																						10,000,000.0
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%
BMS Collaboration Agreement | bempegaldesleukin | Nektar's | Opdivo
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Percentage of sharing development costs	32.50%
BMS Collaboration Agreement | bempegaldesleukin | Nektar's | Opdivo and Yervoy
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Percentage of sharing development costs	22.00%
NKTR-358 | Nektar's
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Deferred revenue												1,300,000									$ 1,300,000
Received upfront and milestone payment						$ 150,000,000.0
Percentage of sharing in Phase 2 development costs			25.00%
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.
AstraZeneca-Kirin
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Percentage of upfront payment, market access milestones, royalties and sales milestones									40.00%
Eli Lilly | NKTR-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)				8,284,600
Sale of stock consideration received				$ 850,000,000.0
Bristol-Myers Squibb | bempegaldesleukin | Research and development
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Reimbursement of expenses																					$ 105,400,000	62,500,000	7,800,000
Bristol-Myers Squibb | BMS Collaboration Agreement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Accounts receivable												24,000,000.0				19,000,000.0					24,000,000.0	19,000,000.0
Bristol-Myers Squibb | 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
Closing date price of common stock (in usd per share)				$ 99.36
Remaining amount allocated to transaction price				$ 1,059,800,000
Potential future development, regulatory and sales milestones				$ 1,800,000,000
Bristol-Myers Squibb | 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%
Amount recognized for granting licenses																						1,059,800,000
Bristol-Myers Squibb | BMS Collaboration Agreement | bempegaldesleukin | Opdivo
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Percentage of sharing development costs	67.50%
Bristol-Myers Squibb | BMS Collaboration Agreement | bempegaldesleukin | Opdivo and Yervoy
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Percentage of sharing development costs	78.00%
Bristol-Myers Squibb | Clinical Trial Agreement | bempegaldesleukin
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Percentage of out-of-pocket costs to be reimbursed by partner								50.00%
Baxalta Incorporated / Takeda
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Potential future additional payments for development milestones																10,000,000.0						$ 10,000,000.0
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 | European Union
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Upfront and milestone payments received from license agreements																					10,000,000.0
Additional upfront and milestone payments recognized from license agreements												10,000,000.0
Eli Lilly and Company | NKTR-358 | Phase 1 Clinical Development
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Received upfront and milestone payment						17,600,000
Eli Lilly and Company | NKTR-358 | Drug Product Development
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Received upfront and milestone payment						6,500,000
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 | NKTR-358 | License
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Received upfront and milestone payment						$ 125,900,000
Eli Lilly and Company | NKTR-358 | License | ASC 606
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenue recognized from contracts with customers		$ 125,900,000
Amgen, Inc.
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Received upfront and milestone payment											$ 50,000,000.0
Amgen, Inc. | ASC 606
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Deferred revenue												4,200,000									4,200,000
Ophthotech Corporation
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Advance payment from collaboration partner							$ 12,700,000
Ophthotech Corporation | Reagent Shipments
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Advance payment from collaboration partner for reagent shipments							$ 10,400,000
Ophthotech Corporation | Minimum Purchase Requirement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Advance payment from collaboration partner																							2,300,000
Ophthotech Corporation | Fovista
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Recognition of remaining deferred revenue																				$ 18,000,000.0
Ophthotech Corporation | Fovista | Novartis Pharma AG
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Received upfront and milestone payment										$ 19,800,000
Bayer Healthcare LLC
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Received upfront and milestone payment																										$ 40,000,000.0
Bayer Healthcare LLC | Performance-based Milestone Payments
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Received upfront and milestone payment																									$ 30,000,000.0
Recognition of remaining deferred revenue																$ 16,800,000
AstraZeneca AB | MOVANTIK and MOVENTIG | Upfront Payment Arrangement
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
Received upfront and milestone payment																							$ 4,600,000
Percentage of upfront payment, market access milestone payments, royalties and sales milestone payments from sublicense agreement retained by our collaboration partner									60.00%
AstraZeneca AB
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												4,000,000.0									4,000,000.0
Terminated Partnership
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Deferred revenue												$ 2,000,000.0									$ 2,000,000.0
Milestone One | Subsequent Event | BMS Collaboration Agreement | bempegaldesleukin | Nektar's
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Potential future additional payments for development milestones																											$ 25,000,000.0
Milestone Two | Subsequent Event | BMS Collaboration Agreement | bempegaldesleukin | Nektar's
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Potential future additional payments for development milestones																											25,000,000.0
Milestone Three | Subsequent Event | BMS Collaboration Agreement | bempegaldesleukin | Nektar's
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Potential future additional payments for development milestones																											$ 75,000,000.0