Collaborative and Other Relationships (Details 3) (Swedish Orphan Biovitrum, USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Summary of activity related to collaboration with Swedish Orphan Biovitrum
Total expense incurred by collaboration	$ 142.9	$ 129.6	$ 78.9
Expense incurred associated with commercial launch capabilities in preparation for the potential product launch	44.7	18.6	0
Total expense reflected within our consolidated statements of income, excluding upfront and milestone payments	187.6	148.2	78.5
Summary of activity related to collaboration, along with an estimate of additional future development expense
Total upfront and milestone payments received from Sobi	5.0
Total expense incurred by Biogen Idec, excluding upfront and milestone payments	468.9
Factors VIII And IX [Member]
Summary of activity related to collaboration, along with an estimate of additional future development expense
Estimate of additional amounts to be incurred by us in development of the lead compound	$ 373.0