Significant Accounting Policies (Details) (USD $) Share data in Millions, except Per Share data, unless otherwise specified	9 Months Ended													1 Months Ended
	Sep. 30, 2011 Regulatory milestones Minimum United States or Europe M	Sep. 30, 2011 Regulatory milestones Minimum United States, Europe or Japan Y	Sep. 30, 2011 Regulatory milestones Maximum United States or Europe M	Sep. 30, 2011 Regulatory milestones Maximum United States, Europe or Japan Y	Sep. 30, 2011 Commercialized milestones	Sep. 30, 2011 New development candidate Minimum M	Sep. 30, 2011 New development candidate Maximum M	Sep. 30, 2011 Phase 1 Minimum Y	Sep. 30, 2011 Phase 1 Maximum Y	Sep. 30, 2011 Phase 2 Minimum Y	Sep. 30, 2011 Phase 2 Maximum Y	Sep. 30, 2011 Phase 3 Minimum Y	Sep. 30, 2011 Phase 3 Maximum Y	Jun. 30, 2008 Genzyme Corporation	Nov. 30, 2010 Alnylam
Research and development revenue under collaborative agreements
Completion period	6	1	12	2		12	18	1	2	1	3	2	4
Pre-specified threshold of sales					$ 1,000,000,000
Deferred revenue recognized upon termination of ssRNAi research program															4,900,000
Equity investment in entity made by strategic alliance partner (in shares)														5
Equity investment in entity made by strategic alliance partner														150,000,000
Equity investment in ISIS by strategic alliance partner, price per share (in dollars per share)														$ 30
License fee amortized														$ 175,000,000