Earnings per Share (Tables)

9 Months Ended

Sep. 30, 2011

Earnings per Share [Abstract]

Basic and diluted earnings per share

	For the Three Months					For the Nine Months
	Ended September 30,					Ended September 30,
(In millions)	2011		2010			2011			2010
Numerator:
Net income attributable to Biogen Idec Inc.	$	351.8	$	254.1		$	934.2		$	765.0
Adjustment for net income allocable to preferred stock		—		(0.5	)		(0.6	)		(1.5	)
Net income used in calculating basic and diluted earnings per share	$	351.8	$	253.6		$	933.6		$	763.5
Denominator:
Weighted average number of common shares outstanding		242.9		239.9			242.3			256.6
Effect of dilutive securities:
Stock options and employee stock purchase plan		0.7		0.9			1.1			0.9
Time-vested restricted stock units		1.6		1.5			1.5			1.4
Market stock units		0.2		—			0.2			—
Performance-vested restricted stock units settled in shares		—		—			—			—
Dilutive potential common shares		2.5		2.4			2.8			2.3
Shares used in calculating diluted earnings per share		245.4		242.3			245.1			258.9

Anti dilutive securities excluded from computation of dilutive earnings per share

	For the Three Months				For the Nine Months
	Ended September 30,				Ended September 30,
(In millions)	2011		2010		2011		2010
Numerator:
Net income allocable to preferred stock	$	—	$	0.5	$	0.6	$	1.5
Denominator:
Stock options		—		4.5		—		4.9
Time-vested restricted stock units		—		1.2		—		1.0
Market stock units		—		—		—		—
Performance-vested restricted stock units settled in shares		—		—		—		—
Convertible preferred stock		—		0.5		0.1		0.5
Total		—		6.2		0.1		6.4