Earnings per Share (Tables)

3 Months Ended

Mar. 31, 2014

Earnings Per Share [Abstract]

Basic and diluted 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