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License and Collaboration Agreements - Additional Information (Detail) - USD ($)			1 Months Ended					3 Months Ended			9 Months Ended		12 Months Ended
	Feb. 13, 2018	Aug. 23, 2017	Apr. 30, 2018	Nov. 30, 2017	Sep. 30, 2017	Sep. 30, 2016	Mar. 31, 2016	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2010	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2015	Dec. 31, 2017
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue recognized in period related to performance in prior periods								$ 18,600,000			$ 64,200,000
Deferred revenue, current portion								15,676,000			15,676,000			$ 18,949,000
Deferred revenue, less current portion								11,410,000			11,410,000			19,021,000
Receivables from customer contracts								13,100,000			13,100,000
Potential future additional development and regulatory milestones								1,700,000,000			1,700,000,000
Accounts receivable, net								31,937,000			31,937,000			5,014,000
Research and development expense								102,895,000	$ 65,714,000		290,653,000	$ 187,032,000
Total revenue								27,762,000	152,928,000		1,153,497,000	212,245,000
Deferred revenue								27,086,000			27,086,000			$ 37,970,000
Development milestones received											$ 12,931,000
AstraZeneca-Kirin [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Percentage of upfront payment, market access milestone payments, royalties and sales milestone payments							40.00%
Nektar's [Member] \| NKTR-358 [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Received upfront and milestone payment					$ 150,000,000
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.
Deferred revenue								11,300,000			$ 11,300,000
NKTR-214 [Member] \| BMS Collaboration Agreement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Research and development expense								1,800,000			2,600,000
NKTR-214 [Member] \| Nektar's [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront and milestone payments received from license agreements			$ 1,000,000,000
NKTR-214 [Member] \| Nektar's [Member] \| Maximum [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Maximum total additional cash payments receivable upon achievement of certain development and regulatory milestones								1,400,000,000
Eligible additional cash payments receivable upon achievement of certain sales milestones								350,000,000
NKTR-214 [Member] \| Nektar's [Member] \| BMS Collaboration Agreement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Global commercialization profits and losses sharing percentage	65.00%
NKTR-214 [Member] \| Nektar's [Member] \| BMS Collaboration Agreement [Member] \| Opdivo [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Percentage of sharing development costs	32.50%
NKTR-214 [Member] \| Nektar's [Member] \| BMS Collaboration Agreement [Member] \| Opdivo and Yervoy [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Percentage of sharing development costs	22.00%
Baxalta Incorporated/Shire [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Potential future additional payments for development and regulatory milestones											10,000,000
Baxalta Incorporated/Shire [Member] \| Hemophilia
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Received under right to sublicense agreement				$ 12,000,000
Baxalta Incorporated/Shire [Member] \| Hemophilia \| European Union [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront and milestone payments received from license agreements											10,000,000
Baxalta Incorporated/Shire [Member] \| Hemophilia \| ASC 606 [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue								900,000			900,000
Baxalta Incorporated/Shire [Member] \| Hemophilia \| ASC 606 [Member] \| European Union [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Development milestones received											10,000,000
Bristol-Myers Squibb [Member] \| Purchase Agreement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Shares issued			8,284,600
Sale of stock consideration received			$ 850,000,000
Bristol-Myers Squibb [Member] \| BMS Collaboration Agreement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Accounts receivable, net								18,800,000			18,800,000
Bristol-Myers Squibb [Member] \| BMS Collaboration Agreement [Member] \| Purchase Agreement [Member]
Collaborative Arrangements And Noncollaborative 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			$ 99.36
Remaining amount allocated to transaction price			$ 1,059,800,000
Potential future development, regulatory and sales milestones								1,800,000,000			1,800,000,000
Bristol-Myers Squibb [Member] \| NKTR-214 [Member] \| Research and Development Expense [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Reimbursement of costs								20,700,000			45,400,000
Bristol-Myers Squibb [Member] \| NKTR-214 [Member] \| BMS Collaboration Agreement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Global commercialization profits and losses sharing percentage	35.00%
Amount recognized for granting licenses											1,059,800,000
Bristol-Myers Squibb [Member] \| NKTR-214 [Member] \| BMS Collaboration Agreement [Member] \| Opdivo [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Percentage of sharing development costs	67.50%
Bristol-Myers Squibb [Member] \| NKTR-214 [Member] \| BMS Collaboration Agreement [Member] \| Opdivo and Yervoy [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Percentage of sharing development costs	78.00%
Bristol-Myers Squibb [Member] \| NKTR-214 [Member] \| Clinical Trial Agreement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Percentage of out-of-pocket costs to be reimbursed by partner						50.00%
Eli Lilly and Company [Member] \| NKTR-358 [Member] \| Phase 1 Clinical Development [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Received upfront and milestone payment					17,600,000
Eli Lilly and Company [Member] \| NKTR-358 [Member] \| Drug Product Development [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Received upfront and milestone payment					6,500,000
Eli Lilly and Company [Member] \| NKTR-358 [Member] \| Maximum [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Potential future additional development and regulatory milestones								250,000,000			250,000,000
Eli Lilly and Company [Member] \| Nektar's [Member] \| NKTR-358 [Member]
Collaborative Arrangements And Noncollaborative 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 [Member] \| Nektar's [Member] \| NKTR-358 [Member] \| Maximum [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Percentage of funding phase 3 development costs on an indication by indication basis borne		25.00%
Eli Lilly and Company [Member] \| Nektar's [Member] \| NKTR-358 [Member] \| Minimum [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Percentage of funding phase 3 development costs on an indication by indication basis borne		0.00%
Eli Lilly and Company [Member] \| License and Maintenance \| NKTR-358 [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Received upfront and milestone payment					$ 125,900,000
Eli Lilly and Company [Member] \| License and Maintenance \| NKTR-358 [Member] \| ASC 606 [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Total revenue									$ 125,900,000			125,900,000
Eli Lilly [Member] \| NKTR-358 [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Percentage of sharing in Phase 2 development costs		75.00%
Amgen, Inc. [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Received upfront and milestone payment										$ 50,000,000
Amgen, Inc. [Member] \| ASC 606 [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue								10,400,000			10,400,000
AstraZeneca AB [Member] \| MOVANTIK and MOVANTIK fixed-dose combination program [Member] \| Upfront Payment Arrangement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Received upfront and milestone payment													$ 385,000,000
AstraZeneca AB [Member] \| MOVANTIK Fixed-dose Combination Program [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Potential contingent payments based on development events								75,000,000			75,000,000
AstraZeneca AB [Member] \| AstraZeneca-Kirin [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Received upfront and milestone payment												$ 4,600,000
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 [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Potential future additional payments for development and regulatory milestones											45,500,000
Deferred revenue								$ 4,500,000			$ 4,500,000