v2.4.0.6

Segment Information (Details)	9 Months Ended
	Sep. 30, 2012 segment
Segment Information (Textual) [Abstract]
Number of reportable segment	1

X

- Definition

Number Of Reportable Segment.

+ References

No definition available.

+ Details

Name:	biib_NumberOfReportableSegment
Namespace Prefix:	biib_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

X

- Details

Name:	biib_SegmentInformationTextualAbstract
Namespace Prefix:	biib_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration