Commitments and Contingencies (Details Textual)	12 Months Ended					12 Months Ended				12 Months Ended		12 Months Ended
	Dec. 31, 2012 USD ($)		Dec. 31, 2011 USD ($)	Dec. 31, 2010 USD ($)	Dec. 31, 2006 FUMADERM and BG-12 USD ($)	Dec. 31, 2010 Biogen Idec Hemophilia USD ($)	Dec. 31, 2012 Biogen Idec Hemophilia USD ($)	Jan. 31, 2007 Biogen Idec Hemophilia USD ($)	Dec. 31, 2012 Weston Facility USD ($)	Dec. 31, 2012 New Cambridge Leases USD ($)	Dec. 31, 2011 New Cambridge Leases USD ($)	Dec. 31, 2012 Eisai [Member] USD ($)	Dec. 31, 2012 Samsung Biosimilar Agreement USD ($)	Dec. 31, 2012 Samsung Biosimilar Agreement KRW
Business Acquisition
Sale Leaseback Transaction Period In Years										15 years		10 years
Future minimum rental commitments	$ 654,800,000	[1]								$ 340,000,000		$ 9,300,000
Cancellation Period of Lease												5 years
Total minimum lease payments									240,000,000
Fair value of contingent consideration	293,900,000		151,000,000	81,200,000				80,000,000
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
Contingent payment achieved in first quarter					15,000,000
Commitments And Contingencies (Textual)
Lease Rent Expense which terminates at various dates	49,000,000		46,200,000	44,800,000
Liabilities associated with uncertain tax positions	71,700,000
Payments to Acquire Equity Method Investments													32,100,000	36,000,000,000
Remaining obligation of financing arrangements										86,500,000	2,200,000		12,500,000	13,500,000,000
Funding commitments, Approximately	11,600,000
Accrued expenses	26,500,000
Cancellable future commitments	440,000,000
Potential future milestone payments commitment, approximately	$ 1,500,000,000

[1] Includes future minimum rental commitments related to leases executed for two office buildings currently under construction in Cambridge, Massachusetts with a planned occupancy during the second half of 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.Includes future minimum rental commitments of $9.3 million related to our lease arrangement with Eisai. The 10 year lease agreement, which is cancellable after 5 years and will become effective in February 2013, gives us the option to purchase the facility. Balances also include remaining total minimum lease payments through 2025, totaling approximately $240.0 million, related to our current corporate headquarters in Weston, Massachusetts, which we expect will be reduced once we relocate our corporate headquarters to Cambridge Massachusetts.For additional information related to these transactions, please read Note 12, Property, Plant and Equipment to these consolidated financial statements.