License and Collaboration Agreements (Details Textual) (USD $)	3 Months Ended								12 Months Ended					3 Months Ended		12 Months Ended						12 Months Ended				3 Months Ended	1 Months Ended	12 Months Ended						12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2007 Up-front Payment Arrangement [Member]	Dec. 31, 2007 Performance milestone payments [Member]	Dec. 31, 2010 Amgen Inc [Member]	Dec. 31, 2011 Amgen Inc [Member]	Dec. 31, 2011 AstraZeneca AB [Member]	Dec. 31, 2011 Bayer Healthcare L L C [Member]	Dec. 31, 2011 Other [Member]	Dec. 31, 2010 Other [Member]	Dec. 31, 2009 Other [Member]	Oct. 31, 2011 Other [Member]	Dec. 31, 2011 Baxter Healthcare [Member]	Dec. 31, 2011 NKTR-118 and NKTR-119 [Member] AstraZeneca AB [Member]	Dec. 31, 2010 NKTR-118 and NKTR-119 [Member] AstraZeneca AB [Member]	Dec. 31, 2009 NKTR-118 and NKTR-119 [Member] AstraZeneca AB [Member]	Dec. 31, 2009 NKTR-118 and NKTR-119 [Member] AstraZeneca AB [Member] Up-front Payment Arrangement [Member]	Dec. 31, 2009 PEGASYS [Member] F Hoffmann La Roche [Member]	Dec. 31, 2011 PEGASYS [Member] F Hoffmann La Roche [Member]	Dec. 31, 2010 PEGASYS [Member] F Hoffmann La Roche [Member]	Dec. 31, 2009 PEGASYS [Member] F Hoffmann La Roche [Member]	Dec. 31, 2011 Nktr 118 [Member]	Dec. 31, 2011 Nktr 118 [Member] AstraZeneca AB [Member]	Dec. 31, 2011 Nktr 119 [Member] AstraZeneca AB [Member]	Dec. 31, 2011 Hemophilia [Member] Baxter Healthcare [Member]	Dec. 31, 2010 Hemophilia [Member] Baxter Healthcare [Member]	Dec. 31, 2009 Hemophilia [Member] Baxter Healthcare [Member]	Dec. 31, 2011 Hemophilia B [Member] Baxter Healthcare [Member]
Deferred Revenue Arrangement [Line Items]
Potential future payments for development milestones									$ 179,300,000							$ 95,000,000															$ 140,000,000			$ 28,000,000			$ 11,000,000
Deferred Revenue															44,200,000		27,400,000					5,700,000						20,500,000
Received upfront payment												40,000,000		50,000,000												125,000,000	31,000,000
Development milestone payments receivable upon achievement of Milestone																	60,000,000
Contingent payments receivable based on development events to be pursued and completed solely by others																																235,000,000	75,000,000
Performance milestone used to reimburse clinical trial costs													20,000,000				10,000,000
License, collaboration and other revenue	6,614,000	16,846,000	6,323,000	6,506,000	32,615,000	30,695,000	31,409,000	29,653,000	36,289,000	124,372,000	36,643,000							15,024,000	5,553,000	4,373,000			2,496,000	107,854,000	25,073,000			5,131,000	5,131,000	214,000				5,646,000	1,701,000	2,055,000
License fee payment receivable																		10,000,000
License fee received current																					5,000,000
License fee receivable in 2013																		3,000,000
License fee receivable in 2015																		2,000,000
Revenue recognized as contractual obligations completed																		5,000,000
Total upfront payments																						9,000,000
Upfront payment recognized																								$ 101,400,000	$ 23,600,000
License and Collaboration Agreements (Textual) [Abstract]
Number of collaboration agreements									2
Upfront payment amortization period										15 months