Segment Information (Tables)

12 Months Ended

Dec. 31, 2012

Segment Reporting [Abstract]

Revenue by product

	For the Years Ended December 31,
	2012						2011						2010
(In millions)	United States		Rest of World		Total		United States		Rest of World		Total		United States		Rest of World		Total
AVONEX	$	1,793.7	$	1,119.4	$	2,913.1	$	1,628.3	$	1,058.3	$	2,686.6	$	1,491.6	$	1,026.8	$	2,518.4
TYSABRI	383.1		752.8		1,135.9		326.5		753.0		1,079.5		252.8		647.4		900.2
Other	—		117.1		117.1		—		70.0		70.0		—		51.5		51.5
Total product revenues	$	2,176.8	$	1,989.3	$	4,166.1	$	1,954.8	$	1,881.3	$	3,836.1	$	1,744.4	$	1,725.7	$	3,470.1

Geographic information

December 31, 2012 (In millions)	U.S.		Europe(1)		Germany		Asia		Other		Total
Product revenues from external customers	$	2,176.8	$	1,216.2	$	409.2	$	93.2	$	270.7	$	4,166.1
Revenues from unconsolidated joint business	$	1,033.3	$	14.3	$	—	$	27.5	$	62.8	$	1,137.9
Other revenues from external customers	$	170.2	$	27.9	$	1.1	$	13.3	$	—	$	212.5
Long-lived assets	$	996.6	$	738.6	$	1.9	$	2.9	$	2.2	$	1,742.2
December 31, 2011 (In millions)	U.S.		Europe(1)		Germany		Asia		Other		Total
Product revenues from external customers	$	1,954.8	$	1,163.3	$	377.5	$	88.7	$	251.8	$	3,836.1
Revenues from unconsolidated joint business	$	878.8	$	29.9	$	—	$	30.7	$	57.2	$	996.6
Other revenues from external customers	$	187.0	$	28.3	$	0.6	$	—	$	—	$	215.9
Long-lived assets	$	1,012.5	$	816.6	$	1.6	$	5.3	$	2.4	$	1,838.4
December 31, 2010 (In millions)	U.S.		Europe(1)		Germany		Asia		Other		Total
Product revenues from external customers	$	1,744.4	$	1,090.7	$	362.4	$	69.0	$	203.6	$	3,470.1
Revenues from unconsolidated joint business	$	906.3	$	95.3	$	—	$	26.0	$	49.6	$	1,077.2
Other revenues from external customers	$	136.0	$	32.6	$	0.5	$	—	$	—	$	169.1
Long-lived assets	$	1,100.3	$	717.4	$	1.5	$	5.4	$	1.6	$	1,826.2