License and Collaboration Agreements - Additional Information (Detail) - USD ($)			1 Months Ended								3 Months Ended			9 Months Ended		76 Months Ended
	Feb. 13, 2018	Aug. 23, 2017	Dec. 31, 2018	Apr. 30, 2018	Jan. 31, 2018	Nov. 30, 2017	Sep. 30, 2017	Aug. 31, 2017	Sep. 30, 2016	Mar. 31, 2016	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2010	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2015
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenue recognized in period related to performance in prior periods											$ 20,500,000	$ 18,600,000		$ 56,600,000	$ 64,200,000
Deferred revenue, current portion			$ 13,892,000								8,392,000			8,392,000
Deferred revenue, less current portion			10,744,000								6,779,000			6,779,000
Potential future additional development and regulatory milestones											1,700,000,000			1,700,000,000
Accounts receivable			43,213,000								41,205,000			41,205,000
Revenue recognized from contracts with customers											29,218,000	27,762,000		80,755,000	1,153,497,000
Deferred revenue			24,636,000								15,171,000			15,171,000
NKTR-358 | Nektar's
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Deferred revenue											3,100,000			3,100,000
BMS Collaboration Agreement | NKTR-214 | Nektar's
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Upfront and milestone payments received from license agreements				$ 1,000,000,000.0
Global commercialization profits and losses sharing percentage	65.00%
Percentage of sharing production costs	65.00%
Maximum total additional cash payments receivable upon achievement of certain development and regulatory milestones				1,400,000,000
BMS Collaboration Agreement | NKTR-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
BMS Collaboration Agreement | NKTR-214 | Nektar's | Opdivo
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Percentage of sharing development costs	32.50%
BMS Collaboration Agreement | NKTR-214 | 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]
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%
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]
Upfront and milestone payments received from license agreements															10,000,000.0
Revenue recognized from contracts with customers					$ 10,000,000.0										10,000,000.0
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 | NKTR-214 | Research and Development Expense
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Reimbursement of expenses											28,300,000	$ 18,900,000		81,400,000	42,800,000
Bristol-Myers Squibb Company | BMS Collaboration Agreement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Accounts receivable											27,700,000			27,700,000
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
Closing date price of common stock (in dollars 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 Company | BMS Collaboration Agreement | NKTR-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%
Amount recognized for granting licenses															$ 1,059,800,000
Bristol-Myers Squibb Company | BMS Collaboration Agreement | NKTR-214 | Opdivo
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Percentage of sharing development costs	67.50%
Bristol-Myers Squibb Company | BMS Collaboration Agreement | NKTR-214 | Opdivo and Yervoy
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Percentage of sharing development costs	78.00%
Bristol-Myers Squibb Company | Clinical Trial Agreement | NKTR-214
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Percentage of out-of-pocket costs to be reimbursed by partner									50.00%
Eli Lilly and Company | NKTR-358 | ASC 606
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenue recognized from contracts with customers								$ 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%
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											5,400,000			$ 5,400,000
AstraZeneca AB | MOVANTIK and MOVENTIG
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Received upfront and milestone payment																$ 385,000,000.0
Amount of post approval study expenses eligible for reimbursement														33.00%
Amount of post approval study expenses eligible for reimbursement, maximum														$ 35,000,000.0
Share of expenses														1,700,000
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%
Baxalta Inc. / Takeda Pharmaceutical Company Ltd. and other
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Deferred revenue											6,000,000.0			6,000,000.0
Potential future additional payments for development milestones											$ 45,500,000			$ 45,500,000