Earnings per Share (Tables)

9 Months Ended

Sep. 30, 2015

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 September 30,				For the Nine Months Ended September 30,
(In millions)	2015		2014		2015		2014
Numerator:
Net income attributable to Biogen Inc.	$	965.6	$	856.9	$	2,715.4	$	2,051.3
Denominator:
Weighted average number of common shares outstanding	232.2		236.2		234.1		236.6
Effect of dilutive securities:
Stock options and employee stock purchase plan	0.1		0.1		0.1		0.1
Time-vested restricted stock units	0.2		0.5		0.3		0.5
Market stock units	0.1		0.2		0.2		0.2
Dilutive potential common shares	0.4		0.8		0.6		0.8
Shares used in calculating diluted earnings per share	232.6		237.0		234.7		237.4