Fair value, as of the acquisition date, of potential payments under the contingent consideration arrangement including cash and shares.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 805

-SubTopic 30

-Section 25

-Paragraph 6

-URI http://asc.fasb.org/extlink&oid=6911189&loc=d3e6408-128476

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 805

-SubTopic 20

-Section 35

-Paragraph 4

-URI http://asc.fasb.org/extlink&oid=6910684&loc=d3e4570-128470

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 805

-SubTopic 30

-Section 25

-Paragraph 7

-URI http://asc.fasb.org/extlink&oid=6911189&loc=d3e6411-128476

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 141R

-Paragraph 68

-Subparagraph g(1)

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 5: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 805

-SubTopic 20

-Section 30

-Paragraph 19

-URI http://asc.fasb.org/extlink&oid=6910544&loc=d3e4243-128469

Reference 6: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 805

-SubTopic 20

-Section 40

-Paragraph 3

-URI http://asc.fasb.org/extlink&oid=6910732&loc=d3e4805-128471

Reference 7: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 141R

-Paragraph 29, 30, 41, 42, 64

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 8: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 805

-SubTopic 30

-Section 50

-Paragraph 1

-Subparagraph (b)(3),(c)(1)

-URI http://asc.fasb.org/extlink&oid=6911361&loc=d3e6927-128479

Reference 9: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 141

-Paragraph 51

-Subparagraph f

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 10: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 805

-SubTopic 30

-Section 25

-Paragraph 5

-URI http://asc.fasb.org/extlink&oid=6911189&loc=d3e6405-128476

Reference 11: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 805

-SubTopic 20

-Section 25

-Paragraph 28

-URI http://asc.fasb.org/extlink&oid=6946367&loc=d3e3642-128468

Reference 12: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 805

-SubTopic 20

-Section 30

-Paragraph 18

-URI http://asc.fasb.org/extlink&oid=6910544&loc=d3e4237-128469

Reference 13: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 141R

-Paragraph 65

-Subparagraph a, b

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 14: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 805

-SubTopic 20

-Section 25

-Paragraph 27

-URI http://asc.fasb.org/extlink&oid=6946367&loc=d3e3629-128468

+ Details

- Definition

For receivables acquired in a business combination, excluding certain loans and debt securities acquired in a transfer (as defined), this element represents the fair value of the receivables acquired, by major class of receivable, such as loans, direct finance leases (as defined), and any other class of receivables.

+ References+ Details

Acquisitions (Details) (USD $) In Millions, unless otherwise specified		9 Months Ended
	Sep. 06, 2011 Biogen Dompe SRL and Biogen Dompe Switzerland GmbH [Member]	Sep. 30, 2010 Biogen Idec Hemophilia [Member]	Sep. 30, 2011 Biogen Dompe SRL [Member]	Dec. 17, 2011 Biogen Idec International Neuroscience GmbH [Member]	Sep. 30, 2011 Biogen Idec International Neuroscience GmbH [Member]
Acquisitions (Textuals) [Abstract]
Cash portion of consideration	$ 152.9			$ 32.5
Value of contingent consideration	42.5			395.0
Fair value of contingent consideration	38.8			81.2	87.1
Receivables purchased as part of acquisition			104.6
Loss recognized upon disposition			1.8
Stock acquired in acquisition				100.00%
Payment to former shareholders of Syntonix Pharmaceuticals		$ 40.0