Earnings per Share (Tables)

3 Months Ended

Mar. 31, 2013

Earnings Per Share [Abstract]

Basic and diluted earnings per share

	For the Three Months Ended March 31,
(In millions)	2013		2012
Numerator:
Net income attributable to Biogen Idec Inc.	$	426.7	$	302.7
Denominator:
Weighted average number of common shares outstanding	236.8		239.8
Effect of dilutive securities:
Stock options and employee stock purchase plan	0.4		0.6
Time-vested restricted stock units	0.8		1.1
Market stock units	0.3		0.3
Dilutive potential common shares	1.5		2.0
Shares used in calculating diluted earnings per share	238.3		241.8