Earnings per Share (Tables)

12 Months Ended

Dec. 31, 2014

Earnings Per Share [Abstract]

Basic and diluted earnings per share

	For the Years Ended December 31,
(In millions)	2014		2013		2012
Numerator:
Net income attributable to Biogen Idec Inc.	$	2,934.8	$	1,862.3	$	1,380.0
Denominator:
Weighted average number of common shares outstanding	236.4		236.9		237.9
Effect of dilutive securities:
Stock options and employee stock purchase plan	0.1		0.3		0.5
Time-vested restricted stock units	0.5		0.8		1.0
Market stock units	0.2		0.3		0.3
Dilutive potential common shares	0.8		1.4		1.8
Shares used in calculating diluted earnings per share	237.2		238.3		239.7