Earnings per Share (Tables)

12 Months Ended

Dec. 31, 2012

Earnings Per Share [Abstract]

Basic and diluted earnings per share

	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