Commitments and Contingencies (Details Textual)	12 Months Ended					12 Months Ended			12 Months Ended								12 Months Ended
	Dec. 31, 2013 USD ($)		Dec. 31, 2012 USD ($)	Dec. 31, 2011 USD ($)	Apr. 02, 2013 TYSABRI product USD ($)	Dec. 31, 2010 Biogen Idec Hemophilia USD ($)	Dec. 31, 2013 Biogen Idec Hemophilia USD ($)	Jan. 31, 2007 Biogen Idec Hemophilia USD ($)	Dec. 31, 2006 Fumapharm AG USD ($)	Dec. 31, 2013 Cambridge Leases USD ($)	Dec. 31, 2013 Samsung Biosimilar Agreement USD ($)	Dec. 31, 2013 Samsung Biosimilar Agreement KRW	Dec. 31, 2012 Samsung Biosimilar Agreement USD ($)	Dec. 31, 2012 Samsung Biosimilar Agreement KRW	Feb. 29, 2012 Samsung Biosimilar Agreement USD ($)	Feb. 29, 2012 Samsung Biosimilar Agreement KRW	Dec. 31, 2013 TECFIDERA USD ($)	Dec. 31, 2013 TECFIDERA Fumapharm AG USD ($)	Dec. 31, 2013 One billion Fumapharm AG USD ($)
Business Acquisition
Term of lease										15 years
Future minimum rental commitments	$ 677,100,000	[1]								$ 340,000,000
Charges recognized to vacate building										27,200,000
Maximum contingent consideration in the form of development and approval milestones								80,000,000	15,000,000
Milestone payments made during period						40,000,000
Additional contingent payment for Biologic License							20,000,000
Additional contingent payment for marketing authorization							20,000,000
Amount paid in cash									220,000,000
Increase to goodwill	35,700,000		48,200,000														15,000,000	25,000,000
Cumulative sales level																			1,000,000,000
Commitments And Contingencies (Textual)
Lease rent expense which terminates at various dates through 2028	56,100,000		49,000,000	46,200,000
Liabilities associated with uncertain tax positions	99,400,000
Payments to Acquire Equity Method Investments											12,400,000	13,500,000,000
Equity Method Investments											23,900,000	25,200,000,000	27,800,000	29,700,000,000	45,000,000	49,500,000,000
Percentage of equity interest to the portion of total capital stock															15.00%	15.00%
Funding commitments, Approximately	14,000,000
Accrued expenses	28,200,000
Cancellable future commitments	424,100,000
Potential future milestone payments commitment, approximately	2,000,000,000
Future contingent payment for the first 12 months					12.00%
Future contingent payment for annual worldwide net sales up to $2.0 billion					18.00%
Future contingent payment threshold					$ 2,000,000,000
Future contingent payment for annual worldwide net sales that exceed $2.0 billion					25.00%

[1] Includes future minimum rental commitments related to leases executed for two office buildings in Cambridge, Massachusetts, which completed construction in July and November 2013. The leases both have 15 year terms and we have options to extend the term of each lease for two additional five-year terms. Future minimum rental commitments under these leases will total approximately $340.0 million over the initial 15 year lease terms.As a result of our decision to relocate our corporate headquarters to Cambridge, Massachusetts, we vacated part of our Weston, Massachusetts facility in the fourth quarter of 2013. We incurred a charge of $27.2 million in connection with this move. This charge represents our remaining lease obligation for the vacated portion of our Weston facility, net of sublease income expected to be received. The term of our sublease to the vacated portion of our Weston facility started in January 2014 and will continue through the remaining term of our lease agreement. For additional information related to these transactions, please read Note 11, Property, Plant and Equipment to these consolidated financial statements.