Significant Accounting Policies (Details) (USD $)	3 Months Ended		6 Months Ended		3 Months Ended	6 Months Ended												1 Months Ended			7 Months Ended	1 Months Ended	7 Months Ended	1 Months Ended		7 Months Ended	6 Months Ended		1 Months Ended					6 Months Ended	12 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013	Mar. 31, 2014 Alnylam [Member]	Jun. 30, 2014 Collaborations and Licensing Agreements [Member] Item Agreement	Jun. 30, 2014 Collaborations and Licensing Agreements [Member] Development milestones - new development candidate [Member] Minimum [Member]	Jun. 30, 2014 Collaborations and Licensing Agreements [Member] Development milestones - new development candidate [Member] Maximum [Member]	Jun. 30, 2014 Collaborations and Licensing Agreements [Member] Development milestones - Phase 1 [Member] Minimum [Member]	Jun. 30, 2014 Collaborations and Licensing Agreements [Member] Development milestones - Phase 1 [Member] Maximum [Member]	Jun. 30, 2014 Collaborations and Licensing Agreements [Member] Development milestones - Phase 2 [Member] Minimum [Member]	Jun. 30, 2014 Collaborations and Licensing Agreements [Member] Development milestones - Phase 2 [Member] Maximum [Member]	Jun. 30, 2014 Collaborations and Licensing Agreements [Member] Development milestones - Phase 3 [Member] Minimum [Member]	Jun. 30, 2014 Collaborations and Licensing Agreements [Member] Development milestones - Phase 3 [Member] Maximum [Member]	Jun. 30, 2014 Collaborations and Licensing Agreements [Member] Regulatory milestones [Member] Minimum [Member]	Jun. 30, 2014 Collaborations and Licensing Agreements [Member] Regulatory milestones [Member] Maximum [Member]	Jun. 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] Target Item	Dec. 31, 2012 AstraZeneca [Member] Agreement entered into in December 2012 [Member] Drug	Jun. 30, 2013 AstraZeneca [Member] Agreement entered into in December 2012 [Member]	Dec. 31, 2012 AstraZeneca [Member] Agreement entered into in December 2012 [Member] ISIS-AR [Member] Item	Jun. 30, 2013 AstraZeneca [Member] Agreement entered into in December 2012 [Member] ISIS-AR [Member]	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]	Jun. 30, 2014 Biogen Idec [Member] Collaborations and Licensing Agreements [Member] Collaboration	Jun. 30, 2014 Biogen Idec [Member] Collaborations and Licensing Agreements [Member] Agreement	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, 2010 GSK [Member] Collaborations and Licensing Agreements [Member]	Jun. 30, 2014 GSK [Member] Collaborations and Licensing Agreements [Member]	Dec. 31, 2013 GSK [Member] Collaborations and Licensing Agreements [Member] Convertible notes guarantee obligation [Member] Drug
Research and development revenue under collaborative agreements
Number of targets																			5	5											3
Upfront fee received																		$ 6,000,000		$ 25,000,000	$ 31,000,000								$ 29,000,000		$ 30,000,000	$ 100,000,000	$ 35,000,000
Upfront fee recorded as deferred revenue																					19,500,000		11,200,000			700,000				12,000,000		100,000,000		7,900,000
Number of drugs collaborative partner may license under the separate research program																			3	3
Number of units of accounting																			4	4		1
Upfront fee recognized																								2,200,000	9,300,000
Upfront fee deferred																					19,500,000		11,200,000			700,000				12,000,000		100,000,000		7,900,000
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 collaborative agreements																											4	4
Number of new development candidates added to collaboration																																			2
Decrease in amortization of upfront payments for 2014						3,300,000
Term of 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	$ 448,000	$ 477,000	$ 9,060,000	$ 2,166,000	$ 7,700,000