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	Dec. 31, 2018	Apr. 30, 2018	Mar. 31, 2018	Jan. 31, 2018	Nov. 30, 2017	Sep. 30, 2017	Sep. 30, 2016	Mar. 31, 2016	Mar. 31, 2019	Mar. 31, 2018	Sep. 30, 2017	Dec. 31, 2010	Sep. 30, 2017	Dec. 31, 2018	Dec. 31, 2015
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue recognized in period related to performance in prior periods											$ 19,600,000	$ 28,000,000
Deferred revenue, current portion			$ 13,892,000								11,092,000					$ 13,892,000
Deferred revenue, less current portion			10,744,000								9,340,000					10,744,000
Potential future additional development and regulatory milestones											1,700,000,000
Accounts receivable			43,213,000								42,894,000					43,213,000
Revenue recognized from contracts with customers											28,222,000	38,018,000
Deferred revenue			24,636,000								20,432,000					24,636,000
NKTR-358 [Member] | Nektar's [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue											$ 5,800,000
BMS Collaboration Agreement [Member] | 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
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] | 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
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]
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] | Hemophilia [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Received under right to sublicense agreement							$ 12,000,000
Baxalta Incorporated/Takeda [Member] | Hemophilia [Member] | Annual Sales Level Milestone [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue recognized from contracts with customers			$ 10,000,000
Baxalta Incorporated/Takeda [Member] | Hemophilia [Member] | European Union [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront and milestone payments received from license agreements					$ 10,000,000
Revenue recognized from contracts with customers						$ 10,000,000						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											$ 28,800,000	$ 2,300,000
Bristol-Myers Squibb [Member] | BMS Collaboration Agreement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Accounts receivable											28,400,000
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 License [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Amount of transaction price allocated to performance obligation								125,900,000
Eli Lilly and Company [Member] | NKTR-358 [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 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] | Phase 1 Clinical Development [Member] | NKTR-358 License [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Amount of transaction price allocated to performance obligation								17,600,000
Eli Lilly and Company [Member] | Drug Product Development [Member] | NKTR-358 License [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Amount of transaction price allocated to performance obligation								$ 6,500,000
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											7,900,000
AstraZeneca AB [Member] | MOVANTIK and MOVENTIG [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
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]
Deferred revenue											6,000,000
Potential future additional payments for development milestones											$ 45,500,000