Earnings per Share (Tables)

6 Months Ended

Jun. 30, 2013

Earnings Per Share [Abstract]

Basic and diluted earnings per share

	For the Three Months Ended June 30,				For the Six Months Ended June 30,
(In millions)	2013		2012		2013		2012
Numerator:
Net income attributable to Biogen Idec Inc.	$	490.7	$	386.8	$	917.4	$	689.5
Denominator:
Weighted average number of common shares outstanding	237.5		239.0		237.2		239.4
Effect of dilutive securities:
Stock options and employee stock purchase plan	0.3		0.6		0.4		0.6
Time-vested restricted stock units	0.6		0.8		0.7		1.0
Market stock units	0.3		0.2		0.3		0.2
Dilutive potential common shares	1.2		1.6		1.4		1.8
Shares used in calculating diluted earnings per share	238.7		240.6		238.5		241.2