Earnings per Share (Tables)

9 Months Ended

Sep. 30, 2012

Earnings Per Share [Abstract]

Basic and diluted earnings per share

	For the Three Months Ended September 30,				For the Nine Months Ended September 30,
(In millions)	2012		2011		2012		2011
Numerator:
Net income attributable to Biogen Idec Inc.	$	398.4	$	351.8	$	1,087.9	$	934.2
Adjustment for net income allocable to preferred stock	—		—		—		(0.6		)
Net income used in calculating basic and diluted earnings per share	$	398.4	$	351.8	$	1,087.9	$	933.6
Denominator:
Weighted average number of common shares outstanding	236.5		242.9		238.3		242.3
Effect of dilutive securities:
Stock options and employee stock purchase plan	0.4		0.7		0.5		1.1
Time-vested restricted stock units	0.9		1.6		1.0		1.5
Market stock units	0.3		0.2		0.3		0.2
Dilutive potential common shares	1.6		2.5		1.8		2.8
Shares used in calculating diluted earnings per share	238.1		245.4		240.1		245.1