Earnings per Share

3 Months Ended

Mar. 31, 2014

Earnings Per Share [Abstract]

Earnings per Share

Earnings per Share

Basic and diluted earnings per share are calculated as follows:

	For the Three Months Ended March 31,
(In millions)	2014		2013
Numerator:
Net income attributable to Biogen Idec Inc.	$	480.0	$	426.7
Denominator:
Weighted average number of common shares outstanding	236.8		236.8
Effect of dilutive securities:
Stock options and employee stock purchase plan	0.1		0.4
Time-vested restricted stock units	0.6		0.8
Market stock units	0.3		0.3
Dilutive potential common shares	1.0		1.5
Shares used in calculating diluted earnings per share	237.8		238.3

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