Significant Accounting Policies (Details) (USD $)	9 Months Ended															12 Months Ended	1 Months Ended			3 Months Ended
	Sep. 30, 2012 item	Sep. 30, 2012 New development candidate Minimum	Sep. 30, 2012 New development candidate Maximum	Sep. 30, 2012 Phase 1 Minimum	Sep. 30, 2012 Phase 1 Maximum	Sep. 30, 2012 Phase 2 Minimum	Sep. 30, 2012 Phase 2 Maximum	Sep. 30, 2012 Phase 3 Minimum	Sep. 30, 2012 Phase 3 Maximum	Sep. 30, 2012 Regulatory milestones Minimum United States or Europe	Sep. 30, 2012 Regulatory milestones Minimum United States, Europe or Japan	Sep. 30, 2012 Regulatory milestones Maximum United States or Europe	Sep. 30, 2012 Regulatory milestones Maximum United States, Europe or Japan	Sep. 30, 2012 Commercialized milestones Minimum	Sep. 30, 2012 Collaborations GSK	Dec. 31, 2011 Collaborations GSK payment	Jan. 31, 2012 Collaborations Biogen Idec Agreement entered into in January 2012	Jul. 31, 2012 Collaborations Biogen Idec Agreement entered into in June 2012	Jun. 30, 2012 Collaborations Biogen Idec Agreement entered into in June 2012	Jun. 30, 2012 Collaborations Genzyme Corporation
Research and development revenue under collaborative agreements
Upfront fee received															$ 35,000,000		$ 29,000,000	$ 12,000,000
Estimated period of performance																	4 years		5 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
Pre-specified threshold of sales														1,000,000,000
Milestone payment recognized															15,000,000					25,000,000
Amount recognized per milestone payment																$ 5,000,000
Number of milestones payments																2