Significant Accounting Policies (Details) (USD $)	3 Months Ended		0 Months Ended	3 Months Ended												1 Months Ended	3 Months Ended					12 Months Ended	1 Months Ended				3 Months Ended			1 Months Ended	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Oct. 31, 2012 GSK Convertible notes guarantee obligation Regulus security	Mar. 31, 2013 Collaborations item	Mar. 31, 2013 Collaborations New development candidate Minimum	Mar. 31, 2013 Collaborations New development candidate Maximum	Mar. 31, 2013 Collaborations Phase 1 Minimum	Mar. 31, 2013 Collaborations Phase 1 Maximum	Mar. 31, 2013 Collaborations Phase 2 Minimum	Mar. 31, 2013 Collaborations Phase 2 Maximum	Mar. 31, 2013 Collaborations Phase 3 Minimum	Mar. 31, 2013 Collaborations Phase 3 Maximum	Mar. 31, 2013 Collaborations Regulatory milestones Minimum	Mar. 31, 2013 Collaborations Regulatory milestones Maximum	Mar. 31, 2013 Collaborations Commercialization milestones	Dec. 31, 2012 Collaborations AstraZeneca item	Mar. 31, 2013 Collaborations AstraZeneca item	Mar. 31, 2013 Collaborations AstraZeneca Maximum item	Jun. 30, 2013 Collaborations AstraZeneca Forecast	Mar. 31, 2013 Collaborations Biogen Idec	Mar. 31, 2012 Collaborations Biogen Idec	Dec. 31, 2012 Collaborations Biogen Idec item	Jan. 31, 2012 Collaborations Biogen Idec Agreement entered into in January 2012	Jun. 30, 2012 Collaborations Biogen Idec Agreement entered into in June 2012	Dec. 31, 2012 Collaborations Biogen Idec Agreement entered into in December 2012 item	Jan. 31, 2013 Collaborations Genzyme Corporation	Mar. 31, 2013 Collaborations Genzyme Corporation	Mar. 31, 2012 Collaborations Genzyme Corporation	Dec. 31, 2012 Collaborations Genzyme Corporation	Feb. 28, 2013 Collaborations GSK	Mar. 31, 2013 Collaborations GSK	Mar. 31, 2012 Collaborations GSK	Dec. 31, 2012 Collaborations GSK
Research and development revenue under collaborative agreements
Number of targets																5									3
Upfront fee received																$ 25,000,000			$ 6,000,000				$ 29,000,000	$ 12,000,000	$ 30,000,000
Number of drugs collaborative partner may license under the separate research program																		3
Number of units of accounting																	4
Revenue earned	43,360,000	23,235,000														9,300,000				3,900,000	1,800,000						25,000,000	16,400,000			9,900,000	2,000,000
Deferred revenue																	15,700,000			59,200,000		62,600,000					3,800,000		3,800,000		17,600,000		19,900,000
Assumed change in estimated selling price of ISIS-STAT3 (as a percent)																10.00%
Percentage by which earned revenue from ISIS-STAT3 would change based on assumed change in estimated selling price																7.00%
Amount by which earned revenue from ISIS-STAT3 would change based on assumed change in estimated selling price																600,000
Number of collaborative agreements																						3
Number of convertible notes no longer guaranteed			2
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
Milestone payment recognized																										$ 25,000,000	$ 25,000,000			$ 7,500,000	$ 7,500,000