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	Jan. 31, 2017	Sep. 30, 2016	Mar. 31, 2016	Jun. 30, 2014	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2010	Sep. 30, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2013	Dec. 31, 2007
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue recognized in period related to performance in prior periods																				$ 95,300,000
Potential future additional payments for development milestones																				1,700,000,000
Deferred revenue, current portion										$ 13,892,000				$ 18,949,000						13,892,000	$ 18,949,000
Deferred revenue, less current portion										10,744,000				19,021,000						10,744,000	19,021,000
Accounts receivable										43,213,000				5,014,000						43,213,000	5,014,000
Development milestones received																				17,331,000
Deferred revenue										24,636,000				37,970,000						24,636,000	37,970,000
Revenue recognized from contracts with customers										39,826,000	$ 27,762,000	$ 1,087,717,000	$ 38,018,000	95,466,000	$ 152,928,000	$ 34,589,000	$ 24,728,000			1,193,323,000	307,711,000	$ 165,436,000
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
ADYNOVATE [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Sales milestone revenue																				10,000,000
BMS Collaboration Agreement [Member] | NKTR-214 [Member] | Nektar's [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Global commercialization profits and losses sharing percentage	65.00%
Percentage of sharing production costs	65.00%
BMS Collaboration Agreement [Member] | NKTR-214 [Member] | Nektar's [Member] | Opdivo [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Percentage of sharing development costs	32.50%
BMS Collaboration Agreement [Member] | NKTR-214 [Member] | Nektar's [Member] | Opdivo and Yervoy [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Percentage of sharing development costs	22.00%
NKTR-358 [Member] | Nektar's [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue										8,300,000										$ 8,300,000
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.
AstraZeneca-Kirin [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Percentage of upfront payment, market access milestones, royalties and sales milestones								40.00%
Baxalta Incorporated/Takeda [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Potential future additional payments for development milestones																				$ 10,000,000
Baxalta Incorporated/Takeda [Member] | Hemophilia
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Received under right to sublicense agreement				$ 12,000,000
Baxalta Incorporated/Takeda [Member] | Hemophilia | ASC 606 [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue										800,000										800,000
Receivables from customer contracts										15,400,000										15,400,000
Baxalta Incorporated/Takeda [Member] | Hemophilia | European Union [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront and milestone payments received from license agreements																				10,000,000
Additional upfront and milestone payments recognized from license agreements										10,000,000
Baxalta Incorporated/Takeda [Member] | Hemophilia | European Union [Member] | ASC 606 [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] | NKTR-214 [Member] | Research and Development Expense [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Reimbursement of expenses																				62,500,000	7,800,000
Bristol-Myers Squibb [Member] | BMS Collaboration Agreement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Accounts receivable										19,000,000										19,000,000
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
Bristol-Myers Squibb [Member] | BMS Collaboration Agreement [Member] | NKTR-214 [Member]
Collaborative Arrangements And Noncollaborative 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 [Member] | BMS Collaboration Agreement [Member] | NKTR-214 [Member] | Opdivo [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Percentage of sharing development costs	67.50%
Bristol-Myers Squibb [Member] | BMS Collaboration Agreement [Member] | NKTR-214 [Member] | Opdivo and Yervoy [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Percentage of sharing development costs	78.00%
Bristol-Myers Squibb [Member] | Clinical Trial Agreement [Member] | NKTR-214 [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
Eli Lilly and Company [Member] | NKTR-358 [Member] | Nektar's [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] | NKTR-358 [Member] | Nektar's [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] | NKTR-358 [Member] | Nektar's [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] | NKTR-358 [Member] | License [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Received upfront and milestone payment					$ 125,900,000
Eli Lilly and Company [Member] | NKTR-358 [Member] | License [Member] | ASC 606 [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue recognized from contracts with customers															$ 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										9,200,000										9,200,000
Ophthotech Corporation [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Advance payment from collaboration partner						$ 12,700,000
Ophthotech Corporation [Member] | Reagent Shipments [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Advance payment from collaboration partner for reagent shipments						$ 10,400,000
Ophthotech Corporation [Member] | Minimum Purchase Requirement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Advance payment from collaboration partner																					2,300,000
Ophthotech Corporation [Member] | Fovista [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Recognition of remaining deferred revenue														18,000,000
Ophthotech Corporation [Member] | Fovista [Member] | Novartis Pharma AG [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Received upfront and milestone payment									$ 19,800,000
Bayer Healthcare LLC [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Received upfront and milestone payment																									$ 40,000,000
Bayer Healthcare LLC [Member] | Performance-based Milestone Payments [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Received upfront and milestone payment																								$ 30,000,000
Recognition of remaining deferred revenue														$ 16,800,000
AstraZeneca AB [Member] | MOVANTIK and MOVENTIG [Member] | Upfront Payment Arrangement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Received upfront and milestone payment																							$ 385,000,000
AstraZeneca AB [Member] | AstraZeneca-Kirin [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue										0										0
Received upfront and milestone payment																					$ 4,600,000	$ 33,000,000
Percentage of upfront payment, market access milestone payments, royalties and sales milestone payments from sublicense agreement retained by our collaboration partner								60.00%
Other [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Potential future additional payments for development milestones																				45,500,000
Deferred revenue										$ 6,400,000										$ 6,400,000