License and Collaboration Agreements - Additional Information (Detail) - USD ($)						1 Months Ended							3 Months Ended			9 Months Ended		12 Months Ended
	Aug. 23, 2017	Sep. 21, 2016	May 30, 2016	Mar. 01, 2016	Sep. 16, 2014	Jan. 31, 2017	Aug. 31, 2016	Mar. 31, 2016	Nov. 30, 2015	Jun. 30, 2014	Jun. 30, 2013	Feb. 29, 2012	Sep. 30, 2017	Dec. 31, 2010	Dec. 31, 2009	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2013	Dec. 31, 2007	Dec. 31, 2017	Dec. 31, 2016
Deferred Revenue Arrangement [Line Items]
Potential future additional payments for development milestones																$ 397,000,000
Liability for refundable upfront payment													$ 12,500,000			12,500,000					$ 12,500,000
AstraZeneca-Kirin [Member]
Deferred Revenue Arrangement [Line Items]
Percentage of upfront payment, market access milestone payments, royalties and sales milestone payments				40.00%
ONZEALDTM (NKTR-102) [Member] | Turkey [Member]
Deferred Revenue Arrangement [Line Items]
Percentage of royalty on net sales			15.00%
Nektar's [Member] | NKTR-358 [Member]
Deferred Revenue Arrangement [Line Items]
Percentage of sharing in Phase 2 development costs	25.00%
Eli Lilly and Company [Member] | NKTR-358 [Member]
Deferred Revenue Arrangement [Line Items]
Received upfront and milestone payment																$ 150,000,000
Percentage of sharing in Phase 2 development costs	75.00%
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%
Regulatory milestones payment, description																A portion of these regulatory 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.
Revenue related to Phase 1 clinical development and drug development activities													1,700,000
Eli Lilly and Company [Member] | NKTR-358 [Member] | License [Member]
Deferred Revenue Arrangement [Line Items]
Received upfront and milestone payment																$ 125,900,000
Revenue related to license and product development activities													125,900,000
Eli Lilly and Company [Member] | NKTR-358 [Member] | Phase 1 Clinical Development [Member]
Deferred Revenue Arrangement [Line Items]
Received upfront and milestone payment																17,600,000
Eli Lilly and Company [Member] | NKTR-358 [Member] | Drug Product Development [Member]
Deferred Revenue Arrangement [Line Items]
Received upfront and milestone payment																6,500,000
Eli Lilly and Company [Member] | NKTR-358 [Member] | Phase 1 Clinical Development and Drug Product Development [Member]
Deferred Revenue Arrangement [Line Items]
Deferred revenue													22,400,000			22,400,000
Eli Lilly and Company [Member] | NKTR-358 [Member] | Maximum [Member]
Deferred Revenue Arrangement [Line Items]
Potential future additional development and regulatory milestones	$ 250,000,000
Indication by indication basis phase 3 percent of development costs	25.00%
Eli Lilly and Company [Member] | NKTR-358 [Member] | Minimum [Member]
Deferred Revenue Arrangement [Line Items]
Indication by indication basis phase 3 percent of development costs	0.00%
AstraZeneca AB [Member] | MOVANTIK and MOVANTIK fixed-dose combination program [Member] | Upfront Payment Arrangement [Member]
Deferred Revenue Arrangement [Line Items]
Received upfront and milestone payment															$ 125,000,000
AstraZeneca AB [Member] | MOVANTIK Fixed-dose Combination Program [Member]
Deferred Revenue Arrangement [Line Items]
Potential contingent payments based on development events													75,000,000			75,000,000
AstraZeneca AB [Member] | AstraZeneca-Kirin [Member]
Deferred Revenue Arrangement [Line Items]
Received upfront and milestone payment								$ 28,000,000								4,600,000
Deferred revenue													0			0
Upfront and milestone payments received by Our collaboration partner under sublicense arrangement				$ 70,000,000
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 [Member] | MOVANTIK [Member]
Deferred Revenue Arrangement [Line Items]
Percentage of post approval study costs to repay					33.00%
Maximum potential reduction in royalties					$ 35,000,000
AstraZeneca AB [Member] | MOVANTIK [Member] | United States [Member]
Deferred Revenue Arrangement [Line Items]
Potential reduction in U.S. royalty rate for repayment					2.00%
Amgen, Inc. [Member]
Deferred Revenue Arrangement [Line Items]
Received upfront and milestone payment														$ 50,000,000
Deferred revenue													15,400,000			15,400,000
Bayer Healthcare LLC [Member]
Deferred Revenue Arrangement [Line Items]
Potential future additional payments for development milestones																50,000,000
Deferred revenue													16,800,000			16,800,000
Payment made to Bayer for cost of Phase 3 clinical trial											$ 10,000,000
Potential future development milestones related to FDA approval																22,500,000
Bayer Healthcare LLC [Member] | Performance-based Milestone Payments [Member]
Deferred Revenue Arrangement [Line Items]
Received upfront and milestone payment																		$ 30,000,000
Bayer Healthcare LLC [Member] | Upfront Payment Arrangement [Member]
Deferred Revenue Arrangement [Line Items]
Received upfront and milestone payment																			$ 40,000,000
Daiichi Sankyo Europe GmbH [Member] | ONZEALDTM (NKTR-102) [Member]
Deferred Revenue Arrangement [Line Items]
Eligible milestone payments receivable upon achievement of regulatory and commercial milestones																60,000,000
Milestone payments to be received upon first commercial sales																10,000,000
Milestone payment to be received upon first commercial sale after final regulatory approval																25,000,000
Milestone payment to be received upon first achievement of annual net sales target																25,000,000
Liability for refundable upfront payment													12,500,000			12,500,000					$ 12,500,000
Daiichi Sankyo Europe GmbH [Member] | ONZEALDTM (NKTR-102) [Member] | European Territory Except Turkey [Member]
Deferred Revenue Arrangement [Line Items]
Percentage of royalty on net sales			20.00%
Daiichi Sankyo Europe GmbH [Member] | ONZEALDTM (NKTR-102) [Member] | Upfront Payment Arrangement [Member]
Deferred Revenue Arrangement [Line Items]
Received upfront and milestone payment							$ 20,000,000
Deferred revenue													3,200,000			3,200,000
Non-refundable portion of upfront payment			$ 7,500,000
Non-refundable portion of upfront payment, subject to conditional marketing approval																12,500,000
Revenue recognized from upfront payment																	$ 3,500,000
Daiichi Sankyo Europe GmbH [Member] | ONZEALDTM (NKTR-102) [Member] | Development Service Obligation [Member]
Deferred Revenue Arrangement [Line Items]
Revenue recognized from upfront payment																600,000
Baxalta Incorporated [Member] | ADYNOVATE (Hemophilia) [Member]
Deferred Revenue Arrangement [Line Items]
Deferred revenue													0			0
Development milestones achieved									$ 10,000,000
Baxalta Incorporated [Member] | ADYNOVATE (Hemophilia) [Member] | European Union [Member]
Deferred Revenue Arrangement [Line Items]
Potential future additional payments for development milestones																10,000,000
Roche [Member] | MIRCERA [Member] | Performance-based Milestone Payments [Member]
Deferred Revenue Arrangement [Line Items]
Received upfront and milestone payment												$ 22,000,000
Roche [Member] | MIRCERA [Member] | Upfront Payment Arrangement [Member]
Deferred Revenue Arrangement [Line Items]
Received upfront and milestone payment												$ 5,000,000
Ophthotech Corporation [Member]
Deferred Revenue Arrangement [Line Items]
Advances from collaboration partner payment						$ 12,700,000
Ophthotech Corporation [Member] | Reagent Shipments [Member]
Deferred Revenue Arrangement [Line Items]
Advances from collaboration partner payment						10,400,000
Ophthotech Corporation [Member] | Minimum Purchase Requirement [Member]
Deferred Revenue Arrangement [Line Items]
Advances from collaboration partner payment						$ 2,300,000
Ophthotech Corporation [Member] | Fovista [Member]
Deferred Revenue Arrangement [Line Items]
Deferred revenue													18,000,000			$ 18,000,000
Ophthotech Corporation [Member] | Fovista [Member] | Novartis Pharma AG [Member]
Deferred Revenue Arrangement [Line Items]
Received upfront and milestone payment										$ 19,800,000
Ophthotech Corporation [Member] | Fovista [Member] | Scenario, Forecast [Member]
Deferred Revenue Arrangement [Line Items]
Deferred revenue																				$ 18,000,000
Bristol-Myers Squibb [Member] | NKTR-214 [Member]
Deferred Revenue Arrangement [Line Items]
Percentage of out-of-pocket costs to be reimbursed by partner		50.00%
License agreement exclusivity expiration date																Sep. 30, 2018
License agreement exclusivity negotiation period																3 months
Other [Member]
Deferred Revenue Arrangement [Line Items]
Potential future additional payments for development milestones																$ 45,500,000
Deferred revenue													$ 5,900,000			$ 5,900,000