Earnings per Share

12 Months Ended

Dec. 31, 2012

Earnings Per Share [Abstract]

Earnings per Share

Earnings per Share

Basic and diluted earnings per share are calculated as follows:

	For the Years Ended December 31,
(In millions)	2012		2011			2010
Numerator:
Net income attributable to Biogen Idec Inc.	$	1,380.0	$	1,234.4		$	1,005.3
Adjustment for net income allocable to preferred stock	—		(0.5		)	(2.0		)
Net income used in calculating basic and diluted earnings per share	$	1,380.0	$	1,233.9		$	1,003.3
Denominator:
Weighted average number of common shares outstanding	237.9		242.4			252.3
Effect of dilutive securities:
Stock options and employee stock purchase plan	0.5		1.0			0.9
Time-vested restricted stock units	1.0		1.3			1.6
Market stock units	0.3		0.3			0.1
Dilutive potential common shares	1.8		2.6			2.6
Shares used in calculating diluted earnings per share	239.7		245.0			254.9

Amounts excluded from the calculation of net income per diluted share because their effects were anti-dilutive were insignificant.

Earnings per share for the years ended December 31, 2012, 2011 and 2010 reflects, on a weighted average basis, the repurchase of 5.8 million shares, 6.0 million shares and 40.3 million shares, respectively, of our common stock under our share repurchase authorizations.