Collaborative Arrangements and Licensing Agreements (Details) (USD $)	3 Months Ended							1 Months Ended		7 Months Ended	3 Months Ended			1 Months Ended	3 Months Ended				1 Months Ended		3 Months Ended				1 Months Ended	3 Months Ended				1 Months Ended	3 Months Ended	1 Months Ended							1 Months Ended		3 Months Ended			1 Months Ended					1 Months Ended	3 Months Ended		1 Months Ended	3 Months Ended	1 Months Ended		3 Months Ended	12 Months Ended	3 Months Ended	12 Months Ended			12 Months Ended	1 Months Ended	3 Months Ended		12 Months Ended				3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013	Mar. 31, 2014 Maximum [Member]	Mar. 31, 2014 Isis SMN [Member]	Mar. 31, 2014 Isis SMN [Member] Open label extension [Member]	Mar. 31, 2014 Period of Performance through June, 2014 [Member] Isis SMN [Member] Phase 2 [Member]	Mar. 31, 2014 Period of Performance through July, 2014 [Member] Isis SMN [Member] Phase 2 [Member]	Jun. 30, 2013 AstraZeneca [Member] Agreement entered into in December 2012 [Member]	Dec. 31, 2012 AstraZeneca [Member] Agreement entered into in December 2012 [Member] Target	Jun. 30, 2013 AstraZeneca [Member] Agreement entered into in December 2012 [Member]	Mar. 31, 2014 Biogen Idec [Member] Collaborations and Licensing Agreements [Member] Collaboration	Mar. 31, 2014 Biogen Idec [Member] Collaborations and Licensing Agreements [Member] Agreement	Mar. 31, 2013 Biogen Idec [Member] Collaborations and Licensing Agreements [Member]	Jan. 31, 2012 Biogen Idec [Member] Agreement entered into in January 2012 [Member]	Mar. 31, 2014 Biogen Idec [Member] Agreement entered into in January 2012 [Member]	Mar. 31, 2014 Biogen Idec [Member] Agreement entered into in January 2012 [Member] Development milestones [Member]	Mar. 31, 2014 Biogen Idec [Member] Agreement entered into in January 2012 [Member] Regulatory milestones [Member]	Mar. 31, 2014 Biogen Idec [Member] Agreement entered into in January 2012 [Member] License fee and substantive milestones [Member]	Oct. 31, 2013 Biogen Idec [Member] Agreement entered into in June 2012 [Member]	Jun. 30, 2012 Biogen Idec [Member] Agreement entered into in June 2012 [Member]	Mar. 31, 2014 Biogen Idec [Member] Agreement entered into in June 2012 [Member]	Mar. 31, 2014 Biogen Idec [Member] Agreement entered into in June 2012 [Member] Development milestones [Member]	Mar. 31, 2014 Biogen Idec [Member] Agreement entered into in June 2012 [Member] Regulatory milestones [Member]	Mar. 31, 2014 Biogen Idec [Member] Agreement entered into in June 2012 [Member] License fee and substantive milestones [Member]	Dec. 31, 2012 Biogen Idec [Member] Agreement entered into in December 2012 [Member] Target	Mar. 31, 2014 Biogen Idec [Member] Agreement entered into in December 2012 [Member]	Mar. 31, 2014 Biogen Idec [Member] Agreement entered into in December 2012 [Member] Development milestones [Member]	Mar. 31, 2014 Biogen Idec [Member] Agreement entered into in December 2012 [Member] Regulatory milestones [Member]	Mar. 31, 2014 Biogen Idec [Member] Agreement entered into in December 2012 [Member] License fee and substantive milestones [Member]	Sep. 30, 2013 Biogen Idec [Member] Agreement entered into in September 2013 [Member]	Mar. 31, 2014 Biogen Idec [Member] Agreement entered into in September 2013 [Member]	Sep. 30, 2013 Biogen Idec [Member] Agreement entered into in September 2013 [Member] Maximum [Member]	Mar. 31, 2014 Biogen Idec [Member] Agreement entered into in September 2013 [Member] Antisense drug for neurological disease [Member] Development milestones [Member]	Mar. 31, 2014 Biogen Idec [Member] Agreement entered into in September 2013 [Member] Antisense drug for neurological disease [Member] Regulatory milestones [Member]	Mar. 31, 2014 Biogen Idec [Member] Agreement entered into in September 2013 [Member] Antisense drug for neurological disease [Member] License fee and substantive milestones [Member]	Mar. 31, 2014 Biogen Idec [Member] Agreement entered into in September 2013 [Member] Other modalities [Member] Pre-specified events [Member]	Mar. 31, 2014 Biogen Idec [Member] Agreement entered into in September 2013 [Member] Other modalities [Member] Development milestones [Member]	Mar. 31, 2014 Biogen Idec [Member] Agreement entered into in September 2013 [Member] Other modalities [Member] Regulatory milestones [Member]	May 31, 2011 Glaxo Smith Kline [Member] Collaborations and Licensing Agreements [Member]	Mar. 31, 2010 Glaxo Smith Kline [Member] Collaborations and Licensing Agreements [Member]	Mar. 31, 2014 Glaxo Smith Kline [Member] Collaborations and Licensing Agreements [Member] Program	Mar. 31, 2013 Glaxo Smith Kline [Member] Collaborations and Licensing Agreements [Member]	Apr. 30, 2014 Glaxo Smith Kline [Member] Collaborations and Licensing Agreements [Member] Subsequent Event [Member]	Mar. 31, 2010 Glaxo Smith Kline [Member] Collaborations and Licensing Agreements [Member] Maximum [Member] Program	Mar. 31, 2014 Glaxo Smith Kline [Member] Collaborations and Licensing Agreements [Member] Pre-specified events [Member]	Mar. 31, 2014 Glaxo Smith Kline [Member] Collaborations and Licensing Agreements [Member] Development milestones [Member]	Mar. 31, 2014 Glaxo Smith Kline [Member] Collaborations and Licensing Agreements [Member] Regulatory milestones [Member]	Mar. 31, 2014 Glaxo Smith Kline [Member] Collaborations and Licensing Agreements [Member] Commercialization milestones [Member]	Dec. 31, 2012 Glaxo Smith Kline [Member] Collaborations and Licensing Agreements [Member] ISIS-TTR [Member]	Mar. 31, 2014 Glaxo Smith Kline [Member] Collaborations and Licensing Agreements [Member] ISIS-TTR [Member]	Mar. 31, 2014 Glaxo Smith Kline [Member] Collaborations and Licensing Agreements [Member] ISIS-TTR [Member] Development milestones [Member]	Nov. 30, 2013 Glaxo Smith Kline [Member] Collaborations and Licensing Agreements [Member] Isis GSK3 [Member] Development milestones [Member]	Mar. 31, 2014 Glaxo Smith Kline [Member] Collaborations and Licensing Agreements [Member] Isis GSK3 [Member] Development milestones [Member]	Nov. 30, 2013 Glaxo Smith Kline [Member] Collaborations and Licensing Agreements [Member] Isis GSK4 [Member] Development milestones [Member]	Apr. 30, 2014 Glaxo Smith Kline [Member] Agreement entered into in September 2013 [Member] Subsequent Event [Member]	Mar. 31, 2014 Achaogen Inc [Member] Collaborations and Licensing Agreements [Member] Drug	Dec. 31, 2006 Achaogen Inc [Member] Collaborations and Licensing Agreements [Member] Series A preferred stock [Member]	Mar. 31, 2014 Achaogen Inc [Member] Collaborations and Licensing Agreements [Member] Maximum [Member]	Dec. 31, 2013 Achaogen Inc [Member] Collaborations and Licensing Agreements [Member] Maximum [Member]	Mar. 31, 2014 Achaogen Inc [Member] Collaborations and Licensing Agreements [Member] Pre-specified events [Member]	Dec. 31, 2009 Achaogen Inc [Member] Collaborations and Licensing Agreements [Member] Pre-specified events [Member]	Dec. 31, 2009 Achaogen Inc [Member] Collaborations and Licensing Agreements [Member] Pre-specified events [Member] Subsequent Event [Member]	Mar. 31, 2004 Alnylam [Member] Collaborations and Licensing Agreements [Member]	Mar. 31, 2014 Alnylam [Member] Collaborations and Licensing Agreements [Member]	Mar. 31, 2013 Alnylam [Member] Collaborations and Licensing Agreements [Member]	Dec. 31, 2013 Alnylam [Member] Collaborations and Licensing Agreements [Member] Phase 3 [Member]	Mar. 31, 2014 Alnylam [Member] Collaborations and Licensing Agreements [Member] Pre-specified events [Member]	Mar. 31, 2014 Alnylam [Member] Collaborations and Licensing Agreements [Member] Development milestones [Member]	Mar. 31, 2014 Alnylam [Member] Collaborations and Licensing Agreements [Member] Regulatory milestones [Member]	Mar. 31, 2014 Alnylam's collaboration with Genzyme [Member] Collaborations and Licensing Agreements [Member]
Collaborative Arrangements and Licensing Agreements
Number of targets									5																3
Upfront fee received								$ 6,000,000	$ 25,000,000	$ 31,000,000				$ 29,000,000						$ 12,000,000					$ 30,000,000					$ 100,000,000									$ 3,000,000	$ 35,000,000									$ 2,500,000														$ 5,000,000
Upfront fee recorded as deferred revenue										19,500,000																				100,000,000																									9,700,000
Number of collaborations											4	4
Period of exclusive rights to use antisense technology																														6 years
Period during which a change in control could result in requirement to refund upfront payment																														6 years		6 years
Milestone payment earned																			10,000,000																															1,000,000		3,000,000	10,000,000	5,000,000												750,000
Cumulative milestone payments earned under collaborative arrangement at period end															16,300,000																																				25,000,000										3,000,000
Payment received related to the amendment of clinical development plan				9,300,000	5,500,000	1,800,000	2,000,000
Number of programs under which drugs are to be developed																																									5			6
Increase in payments that the Company is eligible to receive under collaboration															35,000,000
Maximum amount of payments receivable																78,800,000	150,000,000	303,800,000				49,000,000	130,000,000	259,000,000									60,000,000	130,000,000	260,000,000	90,000,000	35,000,000	55,000,000					142,000,000		1,200,000,000	173,000,000	483,500,000	428,000,000			45,000,000									42,300,000
Next prospective milestone																18,000,000					14,000,000					10,000,000					10,000,000																									4,000,000								375,000
Maximum milestone payments through Phase 2 proof of concept for each program, except TTR and the 5th target																																								20,000,000
Maximum amount of payments receivable per drug under strategic alliance																																																																			3,400,000	1,100,000	2,300,000
Maximum amount of payments receivable per program																											59,000,000	130,000,000	259,000,000
Revenue earned	28,161,000	43,360,000										10,200,000	3,900,000																												3,300,000	9,900,000																						7,700,000	250,000					7,700,000
Percent of total revenue (in hundredths)											36.00%	36.00%	9.00%																												12.00%	23.00%																						27.00%	1.00%
Potential payable on drug development																																																																			3,400,000
Deferred revenue											146,000,000	146,000,000																													10,100,000
Upfront fee in the form of equity securities																																																									1,500,000
Number of drugs to be developed and commercialized																																																								2
Ownership interest percentage (in hundredths)	20.00%		20.00%																																																							10.00%	10.00%
Number of shares held after conversion (in shares)																																																								148,000
Fair value of investment in Achaogen, Inc																																																								1,900,000
Cumulative revenue recognized to date																																																																48,200,000
Payment received in form of securities																																																														$ 500,000