Significant Accounting Policies, Revenue Recognition (Details) (USD $)	3 Months Ended		9 Months Ended														1 Months Ended		7 Months Ended	1 Months Ended				7 Months Ended	1 Months Ended	9 Months Ended			1 Months Ended					3 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014 Collaborations and Licensing Agreements [Member] Category Stage	Sep. 30, 2014 Collaborations and Licensing Agreements [Member] Development milestones - new development candidate [Member] Minimum [Member]	Sep. 30, 2014 Collaborations and Licensing Agreements [Member] Development milestones - new development candidate [Member] Maximum [Member]	Sep. 30, 2014 Collaborations and Licensing Agreements [Member] Development milestones - Phase 1 [Member] Minimum [Member]	Sep. 30, 2014 Collaborations and Licensing Agreements [Member] Development milestones - Phase 1 [Member] Maximum [Member]	Sep. 30, 2014 Collaborations and Licensing Agreements [Member] Development milestones - Phase 2 [Member] Minimum [Member]	Sep. 30, 2014 Collaborations and Licensing Agreements [Member] Development milestones - Phase 2 [Member] Maximum [Member]	Sep. 30, 2014 Collaborations and Licensing Agreements [Member] Development milestones - Phase 3 [Member] Minimum [Member]	Sep. 30, 2014 Collaborations and Licensing Agreements [Member] Development milestones - Phase 3 [Member] Maximum [Member]	Sep. 30, 2014 Collaborations and Licensing Agreements [Member] Regulatory milestones [Member] Minimum [Member]	Sep. 30, 2014 Collaborations and Licensing Agreements [Member] Regulatory milestones [Member] Maximum [Member]	Sep. 30, 2014 Collaborations and Licensing Agreements [Member] Commercialization milestones [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] AccountingUnit Target	Jun. 30, 2013 AstraZeneca [Member] Agreement entered into in December 2012 [Member]	Dec. 31, 2012 AstraZeneca [Member] Agreement entered into in December 2012 [Member] Maximum [Member] Drug	Dec. 31, 2012 AstraZeneca [Member] Agreement entered into in December 2012 [Member] ISIS-AR [Member] AccountingUnit	Jun. 30, 2013 AstraZeneca [Member] Agreement entered into in December 2012 [Member] ISIS-STAT3 License [Member]	Dec. 31, 2012 AstraZeneca [Member] Agreement entered into in December 2012 [Member] ISIS-STAT3 License [Member]	Jun. 30, 2013 AstraZeneca [Member] Agreement entered into in December 2012 [Member] ISIS-STAT3 License [Member]	Mar. 31, 2010 GSK [Member] Collaborations and Licensing Agreements [Member]	Sep. 30, 2014 GSK [Member] Collaborations and Licensing Agreements [Member] Drug	Sep. 30, 2014 Biogen Idec [Member] Collaborations and Licensing Agreements [Member] Agreement	Sep. 30, 2014 Biogen Idec [Member] Collaborations and Licensing Agreements [Member] Strategic Neurology [Member]	Jan. 31, 2012 Biogen Idec [Member] Agreement entered into in January 2012 [Member] ISIS-SMN [Member]	Jun. 30, 2012 Biogen Idec [Member] Agreement entered into in June 2012 [Member] ISIS-DMPK [Member]	Dec. 31, 2012 Biogen Idec [Member] Agreement entered into in December 2012 [Member] Neurology [Member] Target	Sep. 30, 2013 Biogen Idec [Member] Agreement entered into in September 2013 [Member] Strategic Neurology [Member]	Mar. 31, 2004 Alnylam [Member] Collaborations and Licensing Agreements [Member]	Mar. 31, 2014 Alnylam [Member] Alnylam's collaboration with Genzyme [Member]
Research and development revenue under collaborative agreements
Number of targets																		5													3
Upfront fee received																	$ 6,000,000	$ 25,000,000	$ 31,000,000						$ 35,000,000								$ 5,000,000
Upfront fee recorded as deferred revenue																			19,500,000										29,000,000	12,000,000	30,000,000	100,000,000
Number of drugs collaborative partner may license under the separate research program																				3
Number of units of accounting																		4			1
Upfront fee recognized																						2,200,000	9,300,000
Assumed change in estimated selling price (in hundredths)																								10.00%
Percentage by which earned revenue would change based on assumed change in estimated selling price (in hundredths)																								7.00%
Amount by which earned revenue would change based on assumed change in estimated selling price																								750,000
Number of new development candidates added to collaboration																										2
Number of collaborative agreements																											4
Decrease in amortization of upfront payments for 2014					(3,700,000)
Term of collaboration agreement																												6 years				6 years
Number of categories of milestone events					3
Number of stages of life-cycle of drugs					3
Completion period						12 months	18 months	1 year	2 years	1 year	3 years	2 years	4 years
Time to prepare and submit regulatory filings														6 months	12 months
Time to obtain approval														1 year	2 years
Example of sales threshold as milestone event																1,000,000,000
Licensing and royalty revenue	$ 265,000	$ 327,000	$ 9,325,000	$ 2,493,000																														$ 7,700,000